I have a 1996 Montero 4x4 that occasionally jumps out of 2 wheel drive. Th dash light starts to blink and the it starts to make a mechanical sound. I can then stop, put it in neutral, then either put it in 4 wheel high or back into 2 wheel drive and it works fine for a while. Then does it again. It may go 100 miles before it does it again. No consistency. My mechanic thinks it might be electronic or vacuum related, but not the actual transmission that is bad. Any thoughts

Thanks Jerry