I have a 98 D and while driving home one day the vehicle seemed to be down shifting hard when gas was not pressed and just crusing. The vehicle was down shifting hard the revs were up and down, at times really high. The shifting was not smooth from gear to gear. My D is an automatic and this was the first time I felt this happen. The next day the problem was gone and shifting back to normal.

I'm guessing tranny speed sensor or rear diff speed sensor.
Dave