Re: noise from diff??
Yes, it is very hard to help someone with a noise because it can come from so many different things. I just delt with a noise which was coming from the front. At first I wondered if it was a u joint, but after everything I did, I found out I had some loose lug nuts with one still pretty tight. The loose ones allowed the wheel a small amount of movement around the bolt holes which caused the rubbing sound i was hearing. I was lucky for several reasons. No expense to speak of and I found it before the wheel came off.
Do some reading about your drive system and know what type axle you have both front and back. My Jeep has Quadra Drive which is on all the time. There are many components that can give vibration and noise. CV Joints, Hub Bearings, Axle bearings, and u joints on the drive shaft. There are also at least 3 types of front drive shafts, all of which are different as far as u joints. Any of these components can give chatter, squeeks and vibration. Sometimes only when under strain. The right lubricants are important as well, especially when you own something you got used.
If you can better discribe the sounds, keep us informed. Someone will see your response and will help you target the problem.