4wd automatically engaging

Hello all - I'm having a rather unusual problem with my '01 Tacoma 4wd (3.4L, 100k mi). I've talked to a few Tacoma owners about this but they all seem stumped, so I'm asking here.

For some reason, occasionally the 4wd system automatically engages on me while I'm driving - it's happened a number of times and only been while driving on the highway. I can hear a whirring noise coming from the engine bay area that lowers in tone for a few seconds, then ::THUNK:: 4wd is engaged and the dash light comes on. Within a few seconds or so, it turns back off. The 4wd shifter is not touched in any way.

I brought it to my mechanic and he seems to think that the motor inside the differiential is shorting out internally. He disconnected one of the wiring harnesses that connects up to it (the one that enables 4wd I believe) and asked me to drive around for awhile and see if the problem reproduces itself. It didn't for about 2-3 weeks until today.

I'm not really sure what to do, at the bare minimum I'd just like to disable the 4wd completely - I really don't want to spend the money on replacing the front differiential if that is the problem. Any ideas/suggestions??

This shouldn't be to tough to figure out. Don't worry, your "diff" is fine. Find a new mechanic - there is no "motor" inside your differential. When the 4WD button is pushed on the stick, you're completing a circuit to a "solenoid" on the transfer case that in turn engages the 4WD. Your problem is most likely a shorting harness wire, switch or solenoid. Intermittent malfunctions can be frustrating. A little "jiggeling" and an ohmmeter should narrow down the culprit pretty quickly. The good news is, the ultimate fix will shouldn't cost you more than a few nickles and dimes.