I have a 2002 GMC siera 2500HD with a 6.0 L vortec engine and automatic transmission! No matter what gear i put it in it only goes foreward! Even in reverse and nueteral and if u rev the engine in park it boggs out it wants to go foreward but dont cause its in park!!! I checked plungers and wiring inside the oil pan! Can it be the shift silanoids? Help plz anyone! Or do u kno where to get free wiring diagrams or manuals?