I have a vibration from the rear of my Caballero that I thought was due to out of balance wheels,so I took it to Goodyear in Temecula and they would not even try to balance them saying that the problem is in the rear differential.Took it to another place and had them balanced and seemed to be a little better.My question is "can a tech tell if the diff is bad just be raising the vehicle and simply turning a wheel and not even driving it". Bottom line is it is still bothersome at speeds of 50-55.I spun the wheel myself and could not detect a problem!
In my experience, I have never observed the differential to be the cause of vibration. The most likely culprits are the wheel or tire, driveshaft, or axle. You had the wheels balanced, so that will rule out balance or a bent wheel. A broken belt in the tire could cause this, and unless the shop used a "road force" balancer, it might go undetected.
Try rotating the tires front to back for verification. A bent axle can be spotted by spinning the rear wheels off the ground. U-joints and the drive shaft are a little more involved to check. The best way to check for a bad u-joint is to remove the driveshaft and check that move freely and smoothly. If they are tight, or rough, they’ll need replaced.
The only way I have found to check the balance of a driveshaft is to try another one, or have a specialty shop check it. They are balanced at the factory by lightly welding on small tabs of steel. Sometimes they come off. You can usually spot where they were. If the tab came off, it will need to be rebalanced. One more thing to check is the engine and transmission mounts. If they are broken, the driveshaft angle will be incorrect and can cause a vibration. Go ahead and have this checked one more time and make sure that all of these items were covered. If you would like another opinion on the car, a certified technician from YourMechanic can come to your home or office to diagnose the vibration for you.