I worked for Frank Wallace Driveshafts in San Francisco, and we would measure from the bottom of the cup on the differential yoke to the place where the yoke slides into the transmission to get the correct measurement of the shaft. The yoke should ride out of the transmission only about an inch or so, and there are yokes of different lengths. It's a trip how many other things people try to fix before realizing there problem is driveshaft related. A correct measurement will make finding a driveshaft as easy as hauling a tape measure to the parts yard, or you'll know what to ask for from a driveshaft shop.