r/f humming sound

i have a 2000 blazer 2 door, i'm having a problem with a humming sound from the r/f tire. a while ago i was on the hiway and i heard a really loud humming sound from the r/f, i pulled over luckley there was a shop near by. They replaced the wheel bearing , then took it for a test drive and the sound was still there. they figured out that t goes away in 4hi. it only happened going up hills in 2hi. 2 months later i took it into a dealer who looked at it and suggested the vacumn actuater switch. i got that replaced, and now i just hear a soft humming sound all the time but it still goes away in 4hi.

Since you replaced the vacuum switch that is the first thing I would have suggested. You may want to consider replacing the front diff actuator cable as well as I have found from my own experience they can become stretched with frequent use or if a vacuum switch has malfunctioned and caused the diff to lock for an extended period. As far as the humm, is it worse at any particular speeds? Mine humms at around 60km/hr & 120km/hr during deceleration and disappears when 4HI is engaged. A few GM tech's have looked at it and they believe it is the pinion gear, but no one has been able to confirm for sure and I'm not about to start paying for guess work, so I have just lived with it for the past 3 years.

Well before i had the switch replaced the sound changed at diff speeds but now since i had the switch replaced i'll notice it at 25mph and it really dosn't change too much even when on the hiway

Check your front differential fluid level.

I also had the differential fluid changend when i got the switch replacd

Will it hum when doing a left turn at highway speeds?

Will it get louder when you do that?

Well it's pretty consistent on the hiway,but i do notice that it will get a litle louder when driving for long peroids of time. I'm positive it's not bearing because i just had the wheel bearing replaced and when i'm turning at low or moderate speeds the humming is still there. When i kick it into 4hi it's gone back to 2hi and wait 30 sec or so and it's there again.

has to be something wrong with the front drive line then. I would have it inspected.