This is a problem that has been going on for years, and up until recently nothing had changed. My car has a nasty shuddering vibration that starts at 70, gets worse through 75 and gets tolerable again by 80. It can be felt through the wheel and the whole car, yet when you feel the tunnel there's nothing, no heat, no vibration, nothing out of ordinary. The driveshaft and all the U-joints and the cardon joint were overhauled. I'm told it runs true. The rear axle is a 3.08 posi unit. Here's what's funny; the vibration is more pronounced on the right hand side of the road and not as bad on the left or in the passing lane. It doesn't vibrate through acceleration, but it gets slightly worse on deceleration or when loping along. There is a slight whine to the whole set up, it doesn't vary with power being added or not. I had the fluid changed just before I took it on this last long trip, and made sure that the proper limited slip additives were put in. There was a lot of metal and it was dark prior to being dumped. It's only just started getting worse, the vibration point is moving to a lower speed. At lower speeds, there's no driveline clunking or noises, just an easy pull. Turning left from dead stop, there's a hard two or three clunks that sort of feels like the drive shaft is bunching up, but then it releases and the car smoothly moves on. I can't find anything wrong with the steady bearing, the U-joints, or the cardon joints. It all looks okay. So is it the rear axle? Is it time to get it overhauled? Had a similar issue with a Sportwagon, except that when it hit into passing gear the tranny would downshift, the engine would roar but the car would decelerate...back off and it would pull again. What happens when the clutches start to fail?