Loud hum while driving at high speeds, grinding sound at low speeds
 

I have a 98 Blazer 4door 4wd (3button) and I hear grinding almost like a partial engagement on the front right side. I just had the switch replaced so its not that. Any Ideas?

well you could be right on the partial engagement - check and see if there is some vacuum leaking to the engagement diaphram, engagement cable hanging up, if not there then prob time to look internal and see whats worn and hanging up.
You may also may want to jack up that side and rotate the tire by hand, grab the tire top and bottom or both sides and see if you can feel slack in the front hub bearing - check for dragging or ruff feel in the hub and axle along with the engagement

I also found out that it goes away in 4wd and comes back in 2Hi if that helps narrow it down

In 4WD the engagement teeth are fully meshed together - slide gear is not returning tar enough back away from other gear and eating each other .....