Your 3600 should have a two piece driveshaft, closed in front and open in rear. I would pull the rear wheels and open section of the driveshaft and rev engine to 2500rpm; no vibration then all is good. install driveshaft and rev, install wheels and tires and rev. Everything in the rear is now eliminated except tire trueness. We have an old spin balancer which can be used to check out all four wheels and if you have an old timey shop near you they would probably have one but if not the balance/trueness will be trial and error with driving. However, before driving jack up each wheel and set a beverage can (regular or "Lite") as close to the tire without touching and spin the tire. The trueness will be obvious and if you look closely you can also see any runout (wobble). I believe in orderly elimination rather than willy nilly.
