It sounds like you have a tire that iw worn out of round. It can be balanced, but that will not correct it from being out of round.
Also, since you feel the vibration in the seat of your pants, it's probably a rear tire that's messed up otherwise you would feel the vibration in the steering wheel if it was a front tire.
I contantly had the sam problem with a Buick Regal I owned that came with General Tires...In fact, I had the same problem with every vehicle I owned with General Tires. Since I switched to Goodyears about 20 years ago, I have not encounterd the problem. That's not to say it can't happen to Goodyear's but when it happens to every General Tire I owned, I say it's the brand of tire.
The drive shaft balance problem is only noticable at speeds above 70 MPH and since yours happens at lower speeds it's a tire/wheel problem.
...Rich