dbwr,
Better not to use the word axle if you mean differential. The axles are inside the differential. The term axle, to many folks, means rear end/differential. But for this discussion it will be confusing because you can get custom axles to do some things. I am not 100% sure if you are asking about swapping the actual axle shafts or the differential.
I see you have another post about the same issue 7 mo's ago. In that post you said swap "whole axle".
Maybe move away from a HO/drop out style. Look for 3/4T Dana 44 or 14 bolt with the ratio you want. (70's and 80's, 4X4's included) Just measure to get a close fit on the width. The next issue is the suspension you have. You have control arms with coils and a panhard bar. You will need to adapt the donor to this suspension, including E-brake. This does take some effort and skill, a welder, an angle grinder and a magnetic level. That fact drives people to just change R&P. Changing R&P means all new bearings and seals and you can't do it yourself without a lot of tools, study, help/experience and money. Parts hard to find for old drop outs. I would use the chance to get more modern. It is a rock and a hard spot. The donor rear will be way cheaper and more DIY. Good donor $250. I've done it both ways, myself.
The 60's trucks can be a pain in the "rear end". The 4x4's are easy. all leaf springs. You DO need a 8 lug donor or you will have two spare tires to carry.
Also possibly 10 bolt-12 Bolt corporate? Dana 60 (64-77 3/4 T Chevy) ? Could be a ratio problem with some of these.
Biggest complaint about Stovebolts is the lack of speed. If you roll slow, everybody can see how cool your truck is.
