Keep an eye on this behavior. The possibility exists that you may be seeing the beginning of the 722.321 MB "reverseitis". Check the fluid level, change the fluid and the filter ASAP. Don't forget draining the torque converter. There is a control valve in the valve body called the reverser servo start control valve, or something like that. I had the exact same thing happen to me. It required pulling the valve body and cleaning the valve and it's bore. When it's cold, clearances are tighter. When warm, things open up a bit. Keep us posted..
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.