I strongly concur that the trans needs to be electronically diagnosed BEFORE anything's done to it.
My mother's '95 LH New Yorker recently had a trans issue that caused it to go into "Limp-In" Mode (it stayed in 2nd gear and would not upshift). She called the dealer (who'd just installed an engine fan relay) who told her the only real solution (they could guarantee to fix the problem) was a reman transaxle. They would not guarantee that a fluid/filter change would fix it.
I arranged for her to take it to a reputable trans shop. They did a "Quick Learn" with the scan tool as it was sitting in the parking lot. What this does, among other things, is see how long it takes the various clutch packs and bands to apply. The longer it takes for that to happen, the more these friction materials have worn. It's all "pass/fail" sort of stuff.
In her case, with a little over 100K miles on it, they found nothing wrong (wear-wise) and did a full filter change and trans fluid flush. Due to the adaptive nature of that trans, plus the fact that it has never had a fluid change, each Sunday I drove the car to lunch, it felt a little better each time as the new fluid cleaned things up a little bit, progressively, until it now operates better than new.
I don't know what fluid they used, but it obviously has a better friction modifier than the orig fluid did. The hunting (modulated torque converter clutch lock-up) and lower speeds is also gone.
By observation, most of these Chrysler transaxles DO have some quirks about them, but as generally better than many people let on like they aren't good or durable. IF they were as bad as many people claimed, then Thrifty and Dollar Rent-A-Car would have been out of business renting Chrysler products.
The Chrysler transaxles have more sophisticated electronics than GM or even Ford, plus the need for specific trans fluids.
The trans' electronics will adjust the pressures to compensate for slip. IF you put the wrong fluid in them, then it'll try to make things work as best it can, adjusting the pressures something like 400 times/second. With Dexron (without the earlier-spec'd Chrysler additive), it'll keep trying to keep it from slipping . . . . and hammer the friction material from the plates with the pressure adjustments. Chrysler-spec fluid is now available EVERYWHERE, so there's no need to use incorrect fluid.
Some earlier UltraDrives were also prone to suddenly "not move". In this case, the adaptive nature of the electronics would adjust the clutch apply pressures to compensate for clutch pack slip until there was no more friction material on the plates. So it would "be fine" when it was parked and then would not move again when it was restarted.
So, getting the trans checked (electronically) is a key issue! If the Quick Learn shows no wear out of the ordinary range of things, then a full fluid flush and filter change would be the next course of action.
As mentioned, the FAQ at allpar can be highly informative.
Enjoy!
C-BODY