87 and before is made mostly by AMC with some parts made my chrystler. (transmissions and such) 88 and after were made entirely by chrystler. the 87 jeep that I have has 240,000 miles on it and is still going strong.
I think that the transmissions 85 and prior had a problem with the input shaft where it would shatter after about 70-100K miles. After that they are pretty strong. The four wheel drive setup on Jeeps is supurb. When in 4 low the front axle locks into posi-track (track-lock for the ford lovers out there) so it delivers good steady power to all wheels. This is also why you should never try to turn hard while in 4 low.... dragging tires.
Anyway, I love my Jeep(s). they have been nothing but a good strong vehicle for me. and not to hard to work on either.