Don't know anything about those FWD trans, but beefier parts were available for the 904s and 727s IIRC - 4 planet sun gears, heavier duty bands, etc., etc. Someone here might be able to chime in with ideas, part numbers, etc. if the same can be done with the FWD trans.

Also, heat is what kills these things, I realize SLC isn't toasty in the winter, but summer gets into the 90s there doesn't it? You might consider getting an aux cooler for that thing. Maybe the factory had one for those minivans? Or look into getting an aftermarket one that will fit somewhere in front of the radiator/AC condensor.

Maybe even get a trans temp gauge so you can keep tabs on it. We've got a Autometer trans temp gauge on the '01 Durango. It's nice knowing what it runs at. 70 degrees F above ambient seems to be the norm on the highway, but stop-n-go really heats it up. I think I've seen 180-190 degF, and that's WITH the factory aux cooler, but no aftermarket cooler.