My boss has a 1992 4 wheel drive Ford Explorer and she's been complaining that it's been clunking while going up steep hills and acelerating. The noise is not there in 2 wheel drive so I'm assuming that the problem is in the transfer case or the front driveshaft. I haven't heard the "clunking" yet, but today she tried to switch it into 4 wheel drive and although the button lights up suggesting it's in 4 wheel drive it no longer engages. So if it clunks going up hills on load then what are my possible causes. What should I look for?

Transfer Case
Drive Shaft u-Joints
I really don't want to spend too much time on it since it's rather cold outside here. Are these vehicles prone to anything in particular breaking on the front drive train?