1987 blazer 4wd. roar
 

I bought a 1987 blazer 4wd with the 2.8 in it . its a 1owner vehicle with 180000 miles. My problem is when u drive it down the road and let completely off the gas it makes a loud roaring sound from under the blazer. I done checked the rear end and there is no slack in it and its full of gear oil. Ok I found that if you keep the throttle held down just a little from the top and start slowing down it doesn't do it. It only does it when you let off the throttle completely. Has anyone ever had this problem and if so what was the fix to it. Thank you

Differential noise like you're describing can be noticeable on acceleration, maintaining a constant speed, or coasting, (most common is coasting). It's caused by an incorrect backlash adjustment between the ring & pinion gears. If you can hear it, the damage is already done.