Transmission problem.....grinding noise

I need some help here. I have an 89 jeep cherokee 4x4. When I starting driving it drives normal but as soon as I hit 40 mph I hear a grinding noise coming from the transmission. The gear will pull back and I have to let go of the gas to stop the grinding noise. I checked the transmission fluid and it's fine. I have no idea what this could be. Has anyone experienced this problem, what could it be.

Mine did the sametheing and the lever for the transfer case was pulled back a bit... it was partial between 2 and 4 wheel drive.... hope thats all it is for you