02 GMC SIERRA 4.8 120k miles, This truck runs great. I feel i need a Transmission should be flushed out. i had a suck and fill around 85k. not sure what my father did to it in the pass. I done some towing (car.ect) i love this truck and has given us very little problems so far. besides the those dam knox sensor issue. I don’t want to flush the transmission then came and find out later that i need to rebuild it or get a remanufacture one. will ya please give your opinion on this? i trust your answers are just very common sense, 

i break down these parts of the truck or car by 1/3. 1/3 engine, 1/3 trans and the remaining 1/3 everything that is left on the auto is how i view it as for cost.