Serious vibs
1) U-joint(s) shot/grooved trunnions/cap(s) binding.
2) Damaged/imbalanced prop shaft, or wrong driveline angle.
3) Damaged/deformed/worn front yoke, same for rear+ loose nut/bad bearing on pinion. Bad diff/bearings/axle bearings or adjustment.
4) Check rad fan, particularly a clutch type, a bent blade will cause a vibe & not track linear w/rpm....& will fracture the pump shaft if You keep running it up high.
5) Ignition mis/crossfire, check for fine carbon tracking/ionized dust/moisture in the cap, rotor condition.
W/O seeing/hearing/feeling what You're experiencing, those are the best starting points & order I can give You.