Yeah, nothing in your description makes any sense to me. If your clutch was slipping you would see this whenever you accelerated. The harder you tried to speed up the more your clutch would slip. Ahh … sorry, now I see you have an auto transmission and they would have given you a new torque convertor with the tranny. I’d go back to the shop. Truck boys can tell you more but they are going to tell you the same thing. If you have an invoice you can look up the parts and include this in your post to the truck forum.