Have similar problem but at 45mph and 90 mph.
Think mine is propshaft , drivline balance.
Its made worse since I lowered with Ground Force kit.
( changed prop angle )
Ive changed to lightning front shocks as control from ground force was poor to say the least..
I could have a weak trans mount though !!

If you have same problem sugest a few ideas.

1. Add a nut to one of the studs protruding from the coupling
of shaft to diff or shaft to gearbox.
Try different studs.
( this is a classic NVH trick to balance propshafts)
2. Check tires for uneven wear - typical of 60-70 mph problems
flat spotting , curbed etc etc
3. Ajust tire pressures by 2 psi at a time up or down
try and see if has an effect.
I dont recomend keeping at this level
Tires are the most important part of a chassis system and
should always be run at manufacurers pressures.
Trust me I do this for a living !!
4. Leave your shocks unless they are completely worn out
which should be easy to tell ..as you bounce all over the road..

hope this helps ...
cheers