He's right again - hope it does not go to his head....Anyway, a loud bang in a drive train is usually associated with a ruptured u-joint. Blown up a few in my time with hi hp motors...Anyway, you probably have a slightly bent shaft or a twisted/broken yoke. I have done it. In my area we have a dynamite shaft shop that does perfect work - called U-Joint Sales and Service. They work on everything from race cars to dump trucks. Build custom or repair stock. Ask dirt or asphalt racers where they go. Firestone and related type shops typically have personnel good and changing oil, shocks, tires and exhaust systems but not really good at true mechanical things. And CV joints are up there in the specialty group. (U-joints are easy). I would go back to them and complain and tell them to get it right even if they have to sub the job out to a qualified shop. It's your money already spent.....They should have road tested it and fixed it the first time. - Anyway, if you need a lead - ask a racer. Their lives depend on a good driveline.