Hmmmmm..........I have a Mercury Marauder that came with BFG KDWS tires from the factory and it was very smooth until I bought new BFG's....had the same kind of harmonic vibration in the 60-70 MPH range, JUST WHER YA DRIVE...Grrrr!!
We determined it to be tread harmonics and UNFIXABLE!!
Tried everything with no luck and almost all of us Marauder folks had the same results so we moved on to other brands though the field is very limited for these cars!!
I ended up with Kumhos up front and Nittos on the back and it is smooth as silk!! I don't like mixing brands and tread designs but it was my only recourse......I know you want to stay with the "Billboards" and hopefully the truing will fix you up.....Good Luck!!