1995 Chevy Suburban K1500 4WD Problem

I have a 1995 Chevy Suburban K1500 4WD with 234,700 miles. When it is in 2WD it seems like the front axle is dragging or somehow in a bind. If it's put into 4WD (hi or lo) the dragging is even worse. Usually when you back up to switch from 4WD into 2WD, you can hear a clicking noise when the hubs unlock. There is no clicking noise and the hubs seem to stay locked up. Do you have any suggestions as to what might be causing this problem? :? Thanks in advance for any advice.