I had several full size Broncos over the years. As I remember the major difference between the late 1980's models and the later ones was that the earlier ones had manual front wheel drive hubs.
To shift to 4 wheel drive in my 1987 full size Bronco I had to first get out and manually lock the front hubs into the drive position. Once that was complete I could shift the transfer case into either high or low range 4 wheel drive using the lever on the floor.
My 1995 had electric hubs on the front wheels that eliminated the need to get out and manually lock or unlock them. The axles had finer splines than the older manual locking ones and would fail from time to time. At the time Ford made a replacement manual locking assembly that could be used to replace the electric locks.
I doubt Ford still make those kits available, but any good 4 wheel drive shop should be able to advise you as to whether your front axle can be converted and supply parts if it can be done. The axles were from the Dana Corporation and used in many different 4 wheel drive applications of that era.
The other thing to ask the shop is whether when putting the transfer case in neutral is all that's needed to disconnect the rear axle from the transmission. In some models there's enough of a fluid connection remaining (even when the transfer case is in neutral) that the drive shaft will still spin slowly. There may not be enough lubrication within the transfer case and transmission to keep the bearings properly lubricated.
