I bought the truck new last summer took it back within two weeks for XMSN temp going to around 240. they said it is normal this happend just driving on flat grond with truck empty/unloaded. I bought an edge modulue drove with it for about two months, then removed it due to hard shifting. Now at times it feels like it is slipping especially going up hill or when accelerating hard. Once the transmission locked up in I think third gear, and the engine light came on it wouldnt shift manually or automatically. I had to turn it off then restart it to make it work. When I take it into the dealership to get it checked, they say its a truck transmission and should clunk and shift hard. I've rebuilt many a transmission and know when a transmission is slipping. The problem is I am not a pro at rebuiding thansmissions because I dont do it all the time. My question is can I do anything to prove that the transmission has a problem short of dissassembeling it my self and taking bad parts in to GMC.