I have a 98 blazer 4x4. I did have an issue with Transfer case switch failure, (I replaced and now the system is working properly.
However when driving in either 2WD or 4WD High I here a droaning, rumble sound (similar to if you were running big knobby tires. It increases with speed starting at approx 20MPH to highway speed and when you let off the gas it seems louder as the speed winds down .
Thought it might be the tires but they are faily new, so I rotated them anyway (no change). Front drive shaft is not turning (which is correct for running in 2WD). Yet if I put the back on jack stands and in drive at up to 60 MPH with tires in the air do not hear the sound? All fluids are good (truck has 155k) Miles.
This problem seems to have started when my Vacuum switch was bad and front wheel drive was either fully locked or partially locked (due to constant vacuum at acuator. But again that is all fixed and push button 4Whi -2Whi are working. I also looked underneath while wife was accelerating up to 50MPH with both wheels on stands and drive shaft doesn't appear to be wobbling (to the naked eye at least)???
Any suggestions?

PS I shoud add, not able to pin point sound but with wife driving and me listening from the passinger seat, can feel small vibration in floorboard to match sound. and at constant highway speed, has a slowly repeating rhythm to it. Another way to describe sound is like a worn pinion gear in a differentail??? Hmmm maybe it's that??? But again in the air with no load, at 50MPH no droaning sound?
Thanks,
Ken

 

For anyone interested... The sound got worse and I was finnaly able to pin point to the right front. Replaced the RF Hub Bearing assy and she's fine now. It was just a wierd coincidence that a month prior,the 4WD switch went bad and sent her partially into 4WD when it was suppose to be in 2WD (creating a similar sound).