Before you do anything, disconnect the shift rods from the transmission and attempt shifting gears at the transmission to see if the issue is truely in the tranny or just the shifter assembly. Remember one shift lever must be in neural before the second shift lever will be allowed to move.
If you need to remove the tranny:
If you have an engine hoist, you can use that to help manhandle the transmission thru the cab. If not, you can use a 2x4 (4x4) to span the two doors thru the window with a hoist and a rope wrapped around the tranny. There are two bolts on the top of the tranny and two on the bottom accessed thru the clutch inspection cover. Specific steps and details can be found in the Chevrolet Truck Service Manual
here.
Before you do anything, the torque tube must be supported. The 1/2 ton has trunnions at the rear end that enable the torque tube to pivot at the ball joint at the rear of the tranny. If you disconnect the tube from the tranny the tube will be preloaded with the weight of the truck and can cause extreme injuries or even death. You can support the tube with a floor jack or a 2x4 under the tube and resting between the frame rails.
Additionally, clean the front of the torque tube where the universal ball joint and collar slide back onto the torque tube. And when I say clean, I also mean to take some sandpaper and polish that area smooth so the collar slides back easily. Loosening the collar where the seal is also helps.
