if it comes and goes with road speed, usually at multiples of the first speed (say, 30mph smooths out, comes back at 60, etc) check pinion angle. excessive pinion angle (like the 5 degrees nose down recommended for drag racing) can cause vibrations at cruise.

1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)