my a4ld from my 2.9 was not touched other than fluid/filter change at all.. and it still worked fine when i took it out.. i put the 4.0 on in b/c i had it. i've had both of them apart tho. i rebuilt the 4.0 one b/c i didn't know how it was treated from the original owner.. the 2.9 i took apart afterwards b/c i was curious of what it looked like inside w/ 200k miles on it.. looked great.
and the only difference i ever noticed b/t the 4.0 a4ld and the 2.9 a4ld was the 4.0 one had a few more steels and friction plates in the clutch packs.. there could have been other differences, like the torque converter or something, idk... but that's all i ever saw.
my 2.9 a4ld was put back together and is running in a ranger behind a 4.0 now... no problems. not even rebuilt.. just put back together, i'd guess it probably has 270,000 miles on it now. and my 4.0 one has around 45,000(since the rebuild) miles on it and its running great.
i thought it was giving me problems, but it turned out to be the VSS gear stripped.
and the 2wd explorer we have has almost 175,000 miles on it.. w/ regular maintenance and no problems.