Re: Early Bronco 4 speed
The early bronco never had an underdrive 4-speed avaliable. Most 4 speed broncos have been converted using a NP435 or T-18 using the Advanced Adapters' kit.
As for the axles. Any bronco 71 to 77 had a Dana 44 front. 76, 77 had disc brakes. Some broncos (optional) had a 31 spline rear axle shafts vs the 28 spline. When looking for a Dana 20 transfer case, look for the straight shifting case.
Hope this helps,