Check the rear brake shoes, they self adjust in reverse.
It is the automatic air leveling system. If it runs constantly you have a blown rear shock.
Maybe that thumping noise is the sound of another car smashing into the rear of your car.
it sounds like a humming or whinning noise, what year is the truck if its from like 01 or up those trucks have a lot of wheel bearinig problems that some one might misdiagnose as a differential noise or it could be chopped tires making the noise.
close but ur an idiot blazers dont have pinions. they have gear boxes.
The way to tell if you have a bad wheel bearing is when you are driving and go into a hard corner. The wheel that is on the inside of the corner stops making a humming noise. I just replaced my Driver Rear bearing on my 2004 VW Gti
Have you checked your Exhaust Muffler and pipes ? Probably the fuel pump
Its most likely a wheel bearing, I had the same problem about a year ago.
The rear hubs more than likely need grease. If this isn't the case then the axles may need greased or check the tires for broken belts.
Bad Tire or wheel bearing, rotate the tires to the front if the noise follows its the tire if remains in rear its the wheel bearing
try checking the brakes, maybe axel or suspension
Frequently the scrapping noise indicates that the brakes need repairing.