Truck goes into 4WD on it's own?!?

[seamatt]

Hi, I've got a bizarre problem with my truck and haven't the slightest clue what's going on. Driving down the freeway doing about 70 mph last night, i hear this loud whizzing/grinding sound (hard to explain) sounding like it's coming from under the truck. I pull over and slowly slow down, and after my speed is reduced and the sound has gone on for a few seconds, there is kind of a clunk and the 4x4 indicator light comes on the dash. I can tell the truck is in 4HI from the way it was riding/handling. I can shift the lever itself into 4HI, then back to 2WD and it will shift back to 2WD. I would think that maybe I didn't fully shift out of 4HI, but I had not been using 4wd at all yesterday and I've had this issue happen to me 4-5 times within the past couple weeks. Any idea whats going on?!?

My mechanic said something about a 'switch valve' that is on/near the front differential, but the part alone is $310 so I figured I should get a second opinion first.

p.s. My Truck is a 01 reg cab 4x4 5 speed with 145k miles.