I take it that you're out of warranty?
Could be a lot of things. On my Grand Cherokee there's a button on the shift to take it out of overdrive, which you would use to keep it from chasing back and forth between top gear and O/D if you were lugging a load up a hill. Could be that.
If you have the same electronically controlled tranny as mine, you could have a solenoid starting to go. Assuming it's a Chrysler tranny the solenoids are likely "legendary" Mopar electrical components
Could be your torque converter. How many miles on it? I can tell you firsthand that a new torque converter for a GC tranny goes about $1250 including labor
PS--check canyondiver's response to this post on Noreast. It's not a "could-be" answer
