Disco 2 Rear Diff backlash

Gear changes on my auto 1999 D2 TD5 have become quite harsh since fitting a V8 torque converter and there is a definite clunk when coming back on the power at low speed. I had assumed this was a characteristic of the new set up, but it seems slack in the drivetrain is the issue.
The rear propshaft turns maybe 10 degrees before the wheels move suggesting I need a new rear diff.
There is slight play on the front propshaft into the transfer box, but nothing like so pronounced.
As the car was previously owned by a company that manufactures towing equipment and was mapped for heavy towing, I guess this should not be a surprise.
I plan on replacing the rear diff and seeing if this sorts out the problem enough for me not to worry about the transfer box as I would rather not have to replace that as well.
Replacing the diff looks straightforward, I plan to use silicone sealant.
Are there any gotchas, bolts, flanges that will refuse to come off?
Should I replace the cush drive at the same time? It looks OK, but is clearly not new
LR vs OEM vs Aftermarket vs refurb vs scrapyard, thoughts? As that is a variation in price from £850 to £65 this is worth knowing.

I've had the same issue 3 years ago and bit the bullet to buy this https://www.ashcroft-transmissions.co.uk/limited-slip-differentials/ashcroft-atb.html ... after 3 years i consider that it was a wise decission cos i felt it's effect especially in snow or deep mud

Wish I had seen that post last week. I've ordered a 2nd hand stock unit from Paddocks, they have been checked over and I am assured that only those in excellent condition are offered for sale. However, a LSD from Ashcroft would have been worth the extra.