Gmc Sierra will idle but will not drive!

justinblake

Alright I need some help! My father bought his 2007 GMC sierra ext cab 2wd 4.3 new back in 2007 when the new bodystyle come out. It has 80,000 and until a few months ago it has ran fine. He has a company truck and it sat for 6 months he got it out it was running rough he assumed water in the fuel and he poured some heat through it and it seem to fix it. Well a month or so later he drove it and it did it and he added heat and it did not fix it. The truck will idle all day until you put it in gear and it looses power, falls on its face and almost dies. He disconnected the fuel by the throttle body and drained the entire tank because he couldn't siphone it out. New fresh gas did not help. A friend came over and brung a code reader said it was the throttle body, put a new one on did not fix it. Well it eventually made the tranny go out and had it rebuilt, the guy said it look bad like it had been yanked around but it never gets drove? But  still have the problem after the tranny being rebuilt!! Can not figure it out!!! There are no codes that I know of.  So like I said it  will idle pretty decent when you put it in gear it looses all power and falls on its face and almost dies but you can power brake it  and it will try to turn the tires over. Please help this is really stumping everyone 

Thanks Justin