I have a 2000 Dodge Caravan with approx. 100,000 miles. Last year during the fall when it started to get cold, I would reverse out of the driveway, shift into drive, and the van would just sit. If I gave it gas slowly, it would eventually jump into gear and it was fine for the day, unless I parked and it got cold again. I changed the fluid and filter this spring, but the problem persists this fall. I have heard of a “Limp Mode” that the van goes into so that you know that there is a problem but you can still drive it to a repair shop, but if at all possible I would like to know how to diagnose the problem and fix it myself.
Next thing to do is to measure the line pressure when you get this lack of engagement. If the pressure is good in neutral and reverse but there is no pressure until low and/or underdrive clutches engage, you are probably looking at a lazy clutch piston seal or a leaking slip ring.
If you can get to the appropriate feed passage, you could pressuze each circuit with air to see where the leakage is going. Maybe transman618 will chime in; give us the benefit of his experience, and we both can learn.
Hope this helps.