Just recently my '78 Blazer started to shake/vibrate when I get up to around 50mph+ This only seems to happen after it shifts into 3rd and only when I am on the gas. When I let off it there is no more vibration. I am thinking that this is because one or more of my u-joints are going bad. What I can't seem to figue out is how to determine which one it is? What is a good method to figure this out?