I would do a thorough inspection of ALL front end parts, and I mean everything, upper and lower ball joints, and every moving piece of the steering.. My guess is a worn out part allowing a high speed wobble.
double check ALL front end parts,strut rod bushings,uca bushings,lowers too,idler and pittman arms,wheel bearings ect.you have tried differant wheels and tires so i assume these to be ok.double check drive shaft u joints and the balance,if vibes are worse as speed increases=ujoints or balance of shaft.good luck,there are a ton of things to cause vibrations.