1998 Suburban 45 MPH rumble help

I have a 1998 Suburban 1500 4x4 with an automatic transmission and 145,000 miles. I have owned the car since new. It has developed a rumble like a bad bearing this last week. It only happens about 45 MPH going down hill (foot off the gas) in 4th gear or in neutral. I cannot pin point the spot. It is in the back for sure but I cannot tell if it is on one side or the other. My question is could it be an axle bearing or is it in the rear end? If it is an axle bearing are they easy to change? I have checked the driveline joints and they have no movement and show no signs of rust or bearing failures. Any help or ideas would be great. Thanks Dave

You may want to start with the U-Joints on the drive shaft. The down hill thing makes me think that may be it, because it sounds like it only makes noise in a negative stress situation. After that go to wheel bearings and last but certanley NOT $$$ Least the rear end carrier bearings. I had this happen on my 93 Tahoe and it would make a hummiong noise which progressed to a violent clunck every time the ring gear would hit the spot where the broken tooth was missing!
GM actualy fixed it with 110k miles on it because I had it documented as a complaint several times before my GMPP extended warranty was up. The dealers bill showed the total cost at $2600.00 :frown: