Do the easy and cheap things first. New fluid and filter.

Perhaps you should either go back to the dealer, or supply your independent with a quality filter and the transmission fluid you want used. The filters sold on FastLane are good quality, and I'm sure you can get recommendations here for ATF. I can recommend Quaker State 4x4 Synthetic blend Dextron/Mercon-III.

Anytime you have a problem that you can pin down exactly when it started (in your case when the independent changed filter and fluid) the probabilities are that something went wrong at that time.

Good Luck -

Ken300D