Not sure about 90 4x4 Bronco

Ok i just got me a 1990 ford bronco 5.0 4x4 automatic. Its not the first 4x4 have owned its just the first with automatic looking hubs. The question is I pushed the button to turn 4 wheel drive on and it worked but when i turned it off it went off (the light did any way) but when i start going forward or backwards i hear and feel pumping/jarring. Can someone please tell me the problem i just got the thing lol:runaround:

try tapping on the transfer case with a rubber mallet when mine did that i tapped a few times and it worked it happens when the 4by 4 is not engaged often