how do you identify a bad or failing driveshaft?

 

It's jerky when applying clutch

 

There are various types of driveshaft problems. One condition is worn joints to the extent that there is play when moving the driveshaft back and forth. There is also a condition where the joint has no play, but if you change the angle ever so slightly it will bind. The latter is noticeable when riding down a road that causes the suspension to expand and contract, you'll hear a muffled "rubbing" noise.