Re: Rear end gears
 

The easiest thing to do is swapping in 1/2T front and rear axles from a donor 4x4 and selling your axles. Finding one of those for a reasonable price is another issue. Finding one of those with a ratio you want in another issue. If you want a highway driver, you don't buy a 61 GMC V6 4 speed 4X4 and turn it into one. Your truck is worth money the way it is. It's made, and it's ratio is for, four wheeling and power. A work truck, ranch truck, hauler, fun off roading, hunting, fishing with occasional freeway driving. A true original 4X4 GMC, BBV6, two year only hood truck is rare and valuable. Overdrive is another option... or go slower.

I don't know what all the talk about torsion bars/coil springs is all about. It's a 4X4. No torsion bars, no trailing arms, no coil springs.

But if you must: You should be able to use any year 60-66 GMC or Chevy 4X4 and 2x4 (rear) for donor axles with slight modifications. Maybe even more years than that if you want to horse around with it. 60-72 or 73 to infinity and beyond.
Obviously you have to match the ratios front and back. You should be able to use either 1/2T or 3/4T leafs.
Special custom axle shafts or new ring and pinion is an expensive way to go.......real expensive.
I don't think the frame differences/axle widths of 60-62 vs 63-66 will be a big bother in this case. A 2WD GMC donor will have leafs in rear and a Chevy will have coil (most of the time).
If you really don't care about originality and can do mechanical work, the trick thing to do is 70 and up open knuckle/disc brake/205 Xfer case, Chevy/Dodge/GMC. Search internet for more info. Better, stronger, more parts, disc brakes and possibly power steering. I put a 1980 front axle on a 1956 GMC truck. Google 1956 GMC carryall. It will be about the third or fourth picture with saguaro cactus background. Color: 65 Corvette Nassau Blue.