2003 Silverado 4x4 problem

I have a 2003 silverado 4x4 with manual transfer case. The other day while driving in 2hi on a slightly icy road my 4-wheel engaged (approx 70 mph). There was considerable grinding noise and after many tries the 4 wheel kept engaging while my I was set to 2-hi. My local GM dealer removed the tranfer case and haven't found any mechanical damage (they are also at a loss for words as to what happened.)

Questions:
1. What is causing my 4-wheel to engage? i am assuming that a "manual" transfer case isn't as manual as it used to be and some electronics screwed up. If so, please explain.

2. Where did I hear the grinding coming from? What else should I have them inspect for damage?

thanks,
wildman_97

You might have been going fast enouigh that the splines on the shafts in the transfer case could not engage far enough to do real damage, and essentially just resulted in the tips bouncing off of each other and a few shavings being made. I used to work as an intern in the engineering department at one of the big transfer case manufacturers, and got to see a lot of the guts and testing. All of the splines were tapered slightly at the begining to ease engagement, or in your case, to slap the opposing spline out of the way without much damage. Maybe it actually worked!!. There is an adjustment on the shifter that is probably loose or incorrectly set causing the case to pop out of 2hi and try to get into 4hi. This is just like when a manual transmission is out of adjustment and wants to pop out of gear. There are also nylon bushings on the shift forks that wear and cause problems. I would say to check those things. If all else fails you can always use a ratchet strap to keep it in 2hi!! Sorry for the long post but I hope that helps.