My 1993 Chevy 3500 Dual, did the same, I picked up a container of trans-tune and followed the directions on the lable. Drive for about 200 miles, drain tranny fluid, replace trans. filter, fluid, and gasket. I am still driving that truck with same transmission today. Plus I towed a trailer with Harley in it from Key Largo Florida to Connecticut and back. worked in my situation.
Source: Trans-tune at Auto Part Store