FORD MUSTANG NOISE PROBLEM

2001 Ford Mustang • 140,000 miles

Ok, I was driving down the highway and all a sudden I heard a pop and my mustang started to make a humming noise, late for a interview I kept driving and it started making a metal grinding noise.  I changed the rear wheel bearings and seals thinking that could be it because it kinda sounds like a bad bearing.  No luck, still making the same noise, and getting worst.  It's at it's worst when you let off the gas and slowing down, starts whinning and hearing what sounds like metal grinding, if your going up a steep hill it sounds really bad.  Checked front bearings, ujoints, thinking maybe driveshaft problem or something to do with the drive shaft but there is no shaking or shifting problems going on.  Also thaught was kinda weird I jacked the rear end up and put it in drive and it pretty much just sputters, not sure what that's about.