check for vacuum leaks all around the pcv area and the intake. If it dumped a lot of tranny fluid, you should get it checked out immediately. Make sure it stays at the proper level! A low transmission fluid will cause this condition because in reverse, the pressures are higher than in drive. Again, if it lost a lot of fluid, you need to add it back. Truck should be driven for 15 minutes or so and park on level surface with the vehicle idling and check the level.