Probably wear to the friction plates in the clutch pack. (I think the sprag clutch inverting means it all still works but won't set off in 'D', you have to select '1' to pull away & then put it 'D' until you stop again. Ashcroft's site does mention this)
The wear can be accelerated by the kickdown cable being disconnected or incorrectly adjusted and there is also an issue with seals on the input shaft wearing. These look like metal rings round the shaft with stepped ends, best replaced with the teflon ones.
BMW E32 - ZF4HP22 4 - Speed automatic transmission rebuild. (masscom.net)
The 'A' clutch is the one at the front & isn't particularly difficult to remove but be sure to keep everything the way round it came out!
Having the LR factory manual is a big help.
Problem may be that the friction material will be all over and inside everything including the torque converter & the fluid cooler. It's a very fine dust & the cleaning needs to be spotless or it will just get washed into your nice replacement box. Best to replace the TC & it's recommended to replace the cooler & its line/hoses as well.
This is also worth a look, same gearbox with the exception of the very back end & output shaft which is different on the LR. Note he strips the box, not necessary just to do the A clutch.
Chris Cowdery Global Presence
He mentions JP Auto Transmission for parts & I had a few bits for mine from them - incl the teflon seals - when I sorted out all its oil leaks a few years ago. They were excellent but I don't know if they ship overseas. I measured the A clutch friction plates on mine & there was very little wear but it's only one of several clutch packs & there was a lot of the fine dust I mentioned earlier.
ETA I found putting things back on, esp the TC, was much easier when I used an engine crane to hang the box vertically.