I've had this problem for a few months now, and I am exhausted of ideas. I took it in and had an inspection on the suspension (said it was just fine), new tires, balanced, and aligned. Recently changed the front wheel hub assembaly thinking that must be it (they were pretty worn) but the noise remains. The vibration is rythmic and louder as I increase in speed, 4 wheel on the fly and clunk, vibrations gone. 1996 4.3 L blazer.
2 wheel rythmic vibrations; 4X4 clunk, and no more vibration

 

in case anyone was wondering what this problem was caused by, it was an out of balance and failing 4wd drive shaft. I purchased a new one and the problem is gone... yeah!

 

Question, was it your front drive shaft you replaced?