A maximum measurement for a gas vehicle driveshaft or anything turning a high RPM is 54" weld to weld depending on diameter. That would be for a 3" x .090 tube. I've had to cut some down to that length or build a new one in larger diameter if it would fit in the vehicle. Absolute maximum is 60". However your vibration problem could be coming from some other source and sometimes can be really difficult to find. I'd also check the engine and transmission mounts as well as all angularity of the engine, trans and driveline. A common vibration issue is an out of phase rebuild on a driveshaft. If it is even a little bit visable then it it wrong. You've checked the obvious so start checking these suggestions.
