how about your brakes. still using drums up front? i had some bad shakes like this a while back, and after i got my brakes turned (they were dragging and warped) it was much better. the brakes were just catching and slowing the tire down repeatedly, and frequency around 70 mph was just the right amount to shake everything

 

There are discs on front and drum on rear.

 

guess it coudnt hurt to lift the back end and see if the wheels spin smooth?

 

Just got done jacking rear end up and as far a i can tell using a screwdriver on the outside of the wheel and turning them the wheels spin smooth no wobble or anything.