[jholley]

It's a 350. I'm fairly certain the problem is either transmission or rear end. I'm leaning more toward the rear end though because it is the last thing worked on. I pulled a driveshaft out of another truck in the junkyard for troubleshooting purposes. I installed it like it was, old u-joints, carrier bearing and all, it didn't change the vibration at all. Yesterday I put a set of u-joints and a carrier on that old driveshaft and still had exactly the same vibration. I'll have my driveshaft back from the shop today. It will be 5/8" shorter to alleviate the yoke bottoming out and it will have a new slip yoke. I'll update this thread with the results.

It is RPM sensitive though, not speed. I don't have a tach but I can recreate the vib at lower speeds in a different gear. That tells me the problem is somewhere between the motor and the pinion gear. I'm pretty sure it isn't the motor or the driveshaft.