If it's vehicle speed related, it's something in the driveline.
Raise both rear wheels off the ground, transmission in neutral, engine off. Rotate the shaft until the cross is level, grab the shaft near the joint, push up & pull down, then rotate the shaft 90 degrees, push up and pull down again. Should be no perceivable movement, check both joints, same way.

 

I will definately give that a try this weekend that would be a cheap fix as well. Thanks for the input.

 

Does it sound like rushing air, check for clogged catalytic converter, the engine could be pushing air out the non welded sections of exhaust and doughnut gaskets.

This happened to me, much louder the higher the rpm

 

I am have similar problem (1999 Blazer 4.3 L) mine, my sounds like pressure or vacuum, kind of like clogged air restriction first thought was clogged air filter (cheapest to replace) it was dirty but shouldn't be causing what I was hearing. I think mine is clogged converts... My current theory is the noise is real noticeable when the motor is cold, I have a lot of lifter rattle when I accelerate. the whirring noise seems to go away after the motor gets up to operating temp, I have no CEL (no codes) on, fuel mileage is very poor, acceleration is poor, pinging and lifter rattle is you accelerate very fast. I notice that the exhaust is not as loud as it used to be. My conclusion is my converters a clogged. I have been looking at the bolt on replacement form JC Whitney for the bolt on replacement one.
EvanFischer OE Replacement Catalytic Converter 49-State Legal (No CA) - JCWhitney
This is what I am considering. It sounds like you have these similar problems. I hope this will fix my problems.

 

I went ahead and ordered the new catalytic converter assembly should have it sometime this week or next will let you know if it solves my problem similar to your

 

My whirring noise is gone now. I changed the per-converter and the cat-converter over the weekend and now the noise is gone and the power is back like it used to be. I don't know if your problem is the same but it sound similar to mine.

 

mine does the same thing too but the fan is ok. could the 4wd be stuck in hi or lo? :/ how about the CV shafts?