I would try adjusting the linkage. The transmission is the only part that allows the trans to go into park, if the linkage is not adjusted properly (to tight of tolerances) then it cannot travel into park. Put the truck in neutral with e-brake on. Is the needle on the dash in the center of the "N"? If it is not, then you have to crawl under the vehicle, and loosen a nut on the linkage. That is the adjustment side. Put the vehicle into park if you can with the nut loose and the shifter in park. When both are in park, then tighten the jamb nut and then try to shift normally. If works, great, if not, go to transmission shop. This is a GM product and I rarely see any issues with this type of vehicle.
Thursday, November 18th, 2010 AT 2:37 AM