1994 Chevy Blazer Grinding Noise

the best thing to do is see if you can locate the grinding sound. What I am thinking without hearing the vehicle is that it possible be the rear axle bearings. When the vehicle is in 2 wheel drive torque is in the rear end. but in 4 wheel drive the front wheels are taking half the torque. with the more torque in the back {2 wheel} it is putting more strain in the rear bearings. There is also a possibility it could be the rear end itself. That is why you should pinpoint where the noise is comming from.