Is the transmission electronically or vacuum controled? If it is vacuum controlled, you could have a leak. I had a split vacuum hose connecting my transmission to my intake manifold and my tranny wouldn't shift out of first. This is on a Turbo350 though. And, when the engine is running at high rpm without load, it will have very high manifold vacuum. This could mean that when you have the wheels off the ground and run it, it may be overcomming the leak.