Hopefully you are slim enough to be able to get under the truck when it is sat on the floor/road if so no problem. Get under it and loosen off the filler plug it's an allen key socket so needs an allen key, I use one from my kit in a 3/8th drive socket. Just loosen it off so it'll come out quickly and easily once the engine is running.
Prep your plant sprayer by putting a fair amount of ATF in it and pressurising it. Remove the end of the spray arm so you have a bare tube but the trigger is still there and working, point the end of the tube into the ATF bottle and work the trigger until the ATF is coming out. Put the plant sprayer bottle near the drivers door but upright obviously. Put the business end of the spray arm under the truck on a piece of newspaper or summat to keep it clean.
Then get in the truck and run the engine. Once running, run it through all the gear positions slowly but evenly i.e. into D, hold it for a second or two, then three, hold it, then 2 , same thing,then 1, then back the same again then reverse, then park, then neutral etc . Leave it in either P or Neutral. I prefer neutral. Leave the engine running.
Get under the truck. Remove the plug and trigger the sprayer into the box and wait until it runs freely. Release the trigger and then put the plug back in. Stop the engine then check the plug is in tight but not too tight.
Job done.
For a year and a half I had a leak from the gearbox where it joins the torque converter housing so I was doing this a lot. I got so I could do it in about 15 mins with minimum ATF loss. Kept the tools in the door pocket and the plant sprayer in the boot so I could do it at a M way service area.
But seriously, if it is a secondhand box I think you need to change the filter and the fluid. Look at the colour of what comes out. If it is brown not red it needs changing.
Feck it no matter what the colour, for the cost of a filter kit and the ATF, do it anyway. Expect to get ATF dripping down all the time you are under it with the gearbox sump off. This is the ATF that stays in the box.
I am nowhere near a manual or anything but you can find which ATF to put in online.
Plant sprayer like this is what you need doesn't have to be an expensive one.
Amazon product
And you could worse than get this kit.
https://www.jgs4x4.co.uk/discovery-2-automatic-gearbox-filter-kit-with-atf-fluid/
Best of luck with it.
I am assuming you are doing this on your own. If you have a mate or partner it is better but not essential.
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