The noise could either be the t-case or the auto-hubs. Since you just got the Bronco and may not know the condition is why I suggested checking the fluid level and the linkage before taering the t-case apart or taking it to a shop. Alot of drivers seldom use 4LO, and it's often really hard to get into that range. As i posted before, the linkage is often gummed or froze up due to neglect. If that all checks out fine and there's adequate fluid in the t-case, then it could be an internal problem with the shift forks or oil pump. The 13-56's have a problem with the pump mount against the case wearing down, but more often pump failure comes from lack of fluid. As for the auto hubs, again neblect can cause the grease to harden and the hubs not to work. A bad u-joint in the axleshafts or front driveshaft will also makle a racket in 4WD. It just seems to me that a little maintenance first may solve your problem. The 13-56 is a good t-case despite being chain-driven. What is the mileage reading?