i have a 97 4x4 blazer and on the high way at about 70mph if i let off the gas it will make this humming sound with slight vibration but at about 60 it goes away its only between 75 and 65 any ideas

thanks for your time

 

Could be a number of things. Could be something in your rear end maybe, check to see if you have fluid, or maybe even as simple as your lug nuts being loose. Check to see if there tightened really good. It may also be bearings in your rotors is what I'm thinking. How many miles are on the truck? and have the bearings ever been regreased or changed? Does it make this noise or vibration any time else?