93 4x4 high speed vibration

at freeway speeds when "cruzing" not accelerating and not slowing there is a high speed vibration radiating, seems to be coming from the back. How can I narrow this down? U-Joints? The diff is leaking pretty bad and needs new gasket but will the diff vibrate? Please Help.

Start by checking the u-joints, and just because it feels OK on the truck doesn't mean it is. You may have to pull it to be sure.