The noise you're describing is almost always associated with a failing pinion bearing. Check the pinion flange behind the rear universal joint for movement, either up and down or in and out. It shouldn't have a noticeable amount of slack in any direction. You can also jack the vehicle and support both rear axle tubes on sturdy jack stands, run the transmission in high gear, and use a stethoscope or a piece of garden hose to check for noises. Be VERY careful to stay clear of moving parts like the wheels and the driveshaft while doing that kind of testing!
If there's a 2-piece driveshaft, also check the center carrier bearing between the shafts for looseness or bearing failure.
Jerry
