It is not right or common that a cruiser t-case should shift into 4WD by itself. It is quite common however, that it is very difficult to get it out of 4WD if you are on pavement (or rock) and the front hubs are locked in. This is due to "driveline windup", where torsional stress is holding the internal splines together and not allowing the shift coller to slide along them.
You don't mention whether your hubs were locked in or not; knowing whether they were or not might help in diagnosing the problem.
Going by memory, I'm pretty sure you can replace the front output bearing by just taking the nose of the t-case and front output shaft off, without having to pull the entire t-case. I can't remember whether you can also replace the needle bearings that go inside the front of the rear output shaft that the rear of the front output shaft without pulling the whole t-case.
, and this page
from my 76 resto will show you some of the inner workings of the t-case. Good luck and happy cruisin'!