I have a high pitched whining noise when I'm driving 70+ but it goes away when I let off the gas. I thought it had to do with the A/C system but I turned it off and it didn't go away. Any ideas what this might be? I have to blast the radio just to drown out the whine. 70 seems like the magic speed where it starts.

 

I had this same problem and just fixed it. I found a technical bulletin from Jeep addressing this. They redesigned the front drive shaft to get rid of the cv joint. They also made a rear drive shaft with a damper on it that rides aft of the t-case. I complied with the two TSB's linked below and now the driveline is nice and quiet at all speed ranges. Hope it helps!

http://www.wjjeeps.com/tsb/tsb_wj_030500.pdf

http://www.wjjeeps.com/tsb/tsb_wj_0300302a.pdf