My Dakota did the same thing when the clip that holds the boost valve in broke. Pull the pan, and if you see a couple little metal pieces about 1" long and 3/8" wide you need to drop the valve body and replace the clip.

The only symptom mine had was the converter was not locking up. (Before it started dropping out of OD pulling my trailer-that is "limp" mode)

The techs at the local dealership said my trans needed rebuilt. I replaced the clip 30,000 miles ago and everything is still fine.