That isn't true exactly, for these vans. The 4 speed trans had many design flaws, pick up a tran manual someday. The entire 90's Chrysler was releasing trans part upgrades.
If you take an old 4 speed trans from early 90's and put a modern trans repair kit in it, the kit will have all the newer, hardened parts. You may have to machine some parts of the older trans to get everything to fit. Then, it will be reliable - assuming the trans computer is flashed to current code.
It wasn't just the fluid.
Ted