I have a 1985 C10 chevy truck and when I accelerate there is a intermittent vibration under the truck at about 65-70 and it stops as soon as I slow down or if I accelerate and drives like a dream before or after?
Though I would expect the symptoms to appear at any speed I would take a look at the driveshaft center support bearing. Could be exhaust system that manages to vibrate only at that speed.
Have to get it up in the air on a lift or jack stands.
Of course I am just throwing out some guesses.
My thoughts are along the same lines as @Purebred… driveline vibration. In addition to the center support bearing, check all the u-joints and check the pinion flange on the rear axle. None of them should wobble!