Sounds more like a weight came off the driveshaft throwing it out of balance.
Can you duplicate the problem in neutral at the same RPM's as you were running at 50-70 mph?
To visually inspect the balance i would go ahead and remove it and flip it over and look to see if the rubber (sandwhiched between the inner and outer ring) is missing or falling out.
Is this an auto or manual?
1989 Lx Coupe (ex 4 banger) 331 stroker/100 shot
1984 Lx Coupe (ex 6 popper) 310 normal bolt ons
1984 GT hatch 69 351W normal bolt ons
1988 GT hatch 308 (motor still on the stand)
1996 GT stock 4.6 (soon to be supercharged)