There is a vacuum operated modulator on the transfer case that may be partially engaging the front axle assembly. Mine was doing the same thing. I could hear grinding below 40mph. Check the transfer case fluid level. If it was blowing transmission fluid out the breather, it's overfilled. The seal on the transfer input bearing is wearing out. I installed a replacement transmission in my Blazer and it shifted hard from first to second. I read that there could be something worn out in the spool assembly. I sold the Blazer to a friend of mine. I'll probably be rebuilding the old transmission for him in the future.