[texnician01]

Just noticed you have a posi, the best thing then will most likely be the thick gears. When I was looking for a posi it proved to be quite a daunting task, people either didn't want to part with them or they didn't have any.

Also, you will not have to change the whole rearend, just the carrier. The housings and axles are all the same except for length. Your truck would have originally had a narrow axle, the 70 and up trucks had an axle 1 1/2 inches wider, 3/4 on each side. The carriers are interchangeable for any truck 12 bolt and they used them up into the 80's. When changing a carrier and gears, it's not a bad time to rebuild the whole rearend with minimal extra cost.