For sometime now I have noticed what feels like the transmission is slipping in my 2003 Envoy XL. It's hard to explain but it seems to occur under very light throttle on level or when just letting off the throttle when in overdrive, the tach does raise slightly but you hear the exhaust change sound. If you accelerate there is no increase in rpm without speed increasing as if the transmission is slipping. I had this same problem with my 2002 Envoy also. I tow some with the truck and never have a issue with slipping or overheating. I recently serviced all fluids with the exception of the transmission as the pan is rusted and I ordered a new pan. I planned on replacing the 1-2 and 3-4 solenoids along with filter and fluid. Has anyone experienced this or direct me in what is the cause and repair.