Front driveline Intermittant grinding/gear clash 4x4 01 ZR-2

I,m having a problem with my 01 ZR-2 that comes and goes. I,m getting a grinding kind of a gearclash noise comeing from my front end. The lights indicate that I,m in 2-hi. but it sounds to me like it,s trying to go into 4-hi. This noise usually happens when the truck is fully warmed up and starts when i ease off the gas at about 70. I pull over and the noise slows down with speed and will go up with speed. If I come to a complete stop I can feel a disengagement and it stops. I can't make it happen for the mechanics and they have no idea what it is. All I know is it sounds expensive. Hopefully someone here can help.

Thanks,
panamared