96' Yukon SLT 4x4-Strange Drivetrain Noise at 48-56 mph ?

[airforrest]

When driving down the road I get this sound that I can also feel slightly in the Gas pedal...the best way to describe it is a low Groan, or Moan that is intermittent, and lasts for only 5-10 seconds. When it happens, I see no change in the RPM's on the Tach, even though it sounds like it's going into a passing gear. I have ruled out the following items, and it still does it? : I installed new Wheel Hubs/Bearings, New CV Axles, new Brake Light Switch, new U-Joints on the main Driveshaft, and a new Transmission Filter Kit. I have also drained, and inspected the old fluids in the Front, and Read Differentials, Transmission, and Transfer Case for excess Metal shavings, or burnt Fluid smells....nothing seemed abnormal? Could there be a small motor, or solenoid that is triggered at that speed range? I have no check engine, or any other trouble lights on the Dash either. Can anybody else explain this issue, or what it could be?

 

[SunlitComet]

actually get underneath and grab and shake the rear shaft for looseness. possible exhaust groan maybe or just a worn joint or bearing in the tcase or farther back.