what kind of trans and tires are you running. IF you are going to the trouble of adding gears, maybe you should consider a locker and axle shafts if it is all apart, or you could just swap in a D44. If you go with higher gears (4.88+) I would suggest keeping the M20 because the ring gear is 8.875" wide where the D44 is only 8.5" wide. The axles have almost the same spline. The D44 has 30, and the M20 has 29. With one piece axles, and welding your axle tubes to you diff housing would make the M20 just as strong as the D44, maybe stronger.
Found a 85 CJ7 tub in good shape with
minimal rust. The resto continues....