You could have several things contributing to this---

Out of balance crank assy (wrong flywheel, wrong harmonic dampner)

Right dampner but defective

Bad u joint/s

Bad tires causing harmonic thru drive train

Different size rear tires that can cause harmonic thru posi and up drive line

Sheared key in cranksnout (probably as a result of any of the previous and now contributing to the bolt problem

crossed plug wires causing an uneven running engine resulting in harmonics

What you need to do-----is there vibration????? what rpm???? motor shaking???? solid motor mounts?????trans mount????bent drive shaft????wheel????steering wheel shake??? starter bendix wear????