Classic 1995 Range Rover County Classic 3.9 V8 - Idles but stalls out when in Drive or Reverse

Howdy from Texas. I'm new to working on Range Rover and need some help. My uncle sent me his 1995 Range Rover County Classic with a 3.9 gas V8 (174,000 miles) for me to fix. It's 4wd with an automatic transmission. I work on motorcycles so he's thinking I might be able to get this going but am not having much luck. Full history of the situation with this car:

He bought it used on ebay and it idled low and didn't run very well so he had a shop here in Texas look at it. They replaced the fuel pump, "throttle potentiometer retrofit kit", new fan clutch, drive belt tensioner assembly, air conditioning compressor and receiver/drier, water pump, new battery, new distributor with cap/rotor, new spark plugs, spark plug wires, and did some sort of fuel injection cleaning/flush for a grand total of $4400. It ran fine for a day and then wouldn't stay idling. He let it sit a few months out of frustration and then had it towed to my garage where I work on motorcycles but I can't seem to figure out what's wrong with it. It would crank up, idle rough, and then shortly stall out. If I apply throttle, it wants to backfire and then stall out. I noticed it was shooting unspent fuel out of the exhaust. I suspected the timing was off so I checked all 8 spark plugs for spark with an inline spark tester (they all have spark) and then made a mark on the distributor and began moving it's position from left to right seeing if the timing change made any difference with how it ran. The original position I marked on the distributor was the only position the engine would start at all so I assume the shop that did the work installed the distributor correctly and properly timed the engine.

I have it now where it will consistently start and idle without stalling out but as soon as I put the automatic gearbox into drive or reverse, it wants to immediately stall out. What's weird is that if I press the brake pedal while in drive or reverse while accelerating the engine will stay running (it runs rough and wants to die but will stay running). Also, the gear shift lever is really hard to engage. Any idea what's causing this? I don't think there are any issues with bad gas, bad fuel pump, or bad injectors as it will idle all day but stalls out when in drive or reverse or sometimes while revving the engine. When it is idling and I attempt to give it throttle while in park, sometimes it accelerates and sometimes it hesitates/backfires a little. I have a shop manual but I'm not used to working on these old British cars and it's not really much help. I'm used to plugging things into a code reader and troubleshooting from there lol.

I appreciate any help

Have you checked the timing on the dizzy?
Anywhere between 6° and 10° btdc is a good place to start

Brake pedal pressed affects it? Check for a vacuum pipe leaking.

Air leaks on the intake of these can cause problems - in particular when the air is able to enter the engine after the Air Flow Meter so is unmetered.
Check all hoses for security & any splits.
If the plenum chamber and ram housing have been fitted without sealant this can also allow air leaks. Spray around with something like 'EasyStart' or an aerosol of 'Clutch & Brake Cleaner'. If the rpm increases then you have a leak.

This engine will have a 'stepper motor' which is on the rear of the plenum chamber.
This is an electronically operated automatic air valve and opens & closes itself to increase/decrease extra air to prevent the engine stalling when you engage D or R.
They can gum up. Can be removed & cleaned. Think there's a 'how to' somewhere on Landyzone if you use the 'search' at the top.

Best not to assume the timing is set correctly.
Remove the distributor cap. Note where No1 lead is.
Turn the rotor arm gently with your fingers until it stops, then release. It should return smartly to its original position.
Check that the rotor arm is a good quality item and NOT one of the type with the metal strip riveted to it.
If you want to remove the rotor arm you MUST press down on the reluctor (eight pointed star shaped thing under the rotor arm) as you pull or there is a good chance you will disengage the auto advance/retard mechanism (weights & springs) in the bowl of the dizzy. If you do it likely won't run at all. Turning the rotor arm with your fingers as above will point this up as it won't go back to its original position.
Remove the plug from No1 cylinder. The front cylinder of the left hand bank (as viewed sitting in the drivers seat facing forward).
Remove the left hand rocker cover.
Put a long rod in No1 plughole so it rests on the piston.
Turn the engine in the normal direction of rotation until the rod is at the top of its travel & the valves are both closed. Top of the compression stroke. If you go past this point then turn the engine backwards until the piston is about halfway or further down & then turn in normal direction again.
That is TDC.
You may find that the TDC mark on the crank pulley more-or-less aligns with the pointer.
The rotor arm should be pointing at No1 lead's position.
If it isn't then you will need to remove & reposition the dizzy. Note that the dizzy will rotate slightly as you withdraw it as its drive gear is at an angle. You will need to position the rotor arm to take account of this as you refit it.
Check that the rest of the leads are correctly located & in the correct order & that they aren't touching.

Final adjustment on these best set by road test.
With everything in position, accelerate. You will likely hear a tinny rattle. Stop, loosen the bolt & turn the dizzy slightly. Tighten the bolt. Repeat until it JUST makes the tinny noise at hard throttle.
Then check with a timing light to see where the pointer is pointing.

The difficulty moving the gear selector lever could be the cable is damaged, gummed up or incorrectly routed. Had the last one on mine after I'd removed & refitted the transfer box, suddenly became very stiff.

If the shop manual you have isn't the Land Rover factory one then either you or the owner need to get it.
For these it's a CD.

Mmmmmm....... intriguing.
If you didn't sort this yet, try checking out the bits that make up the idle controls. So, after making sure you don't have any inlet leaks check the ignition timing as already advised but also check the vac advance is working properly. Stepper (idle control valve or inlet air control) is easy to remove, clean and test. Don't go putting 12v across it though, I believe it works on 5v from ECU. Don't forget to check the bypass air hose that runs at the back of the plenum from the stepper to the throttle body, they do get a bit crumbly with age.

Check the throttle pot - base voltage is the key. If the base voltage is too high, idle stabilisation might not be working. Do a search in the Engines, V8 section, I've posted on throttle pot checks before.

I'm a bit rusty these days as sold my classic but iirc there should be an input to tell the ecu the car is in gear, this affects idle stabilisation as does the speed sensor but that won't have any effect until the car is moving. HTH good luck.