360 engine vibration at ~3000rpm

This does not affect your vibration, but in one post you said someone hit the U bolts with an impact. you guys REALLY need to buy and READ your factory service manuals. The leaf spring U bolt torque is low. It's only 45 LB FT. The reason is so the springs can actually "work" when the suspension jounces and rebounds. Over tightening those U bolts will result in the suspension binding and riding rough. You need to re-torque those to the proper spec.