I get the same vibration too, I figured it could be the angle of the front drive shaft? It kinda hops on the highway when foot is off the gas. When $ allows I want to get a CV drive shaft, hopefully that will fix the vibration.
2001 Ironman Daily Driver... 3.5" suspension lift (OME912 springs and 1" spring spacer), ball joint flip, 1.5" front diff. drop, 33"x12.5 TrXus MT, 16x10 Eagle Alloy rims, Interceptor, PV muffler, K&N air filter, Alpine Supercharger, Bilstein shocks, and some trimming.
2000 Ironman Project LS-1 VX... very slow progress but someday....... ohhhhh someday......