The frequency of the "rumble" was slower than the motor speed and faster than the drum speed. I concluded it was the rollers -- specifically, the lower, rear roller. Since I was going to replace one and planned to pull the drum, I decided to replace all 4 rollers. I also replaced the drum belt while I had access (although it appeared to be in good shape). I did not replace the roller shafts. I figured the hardened steel shafts should be good for life.
I took the dryer apart from the back side. I later noticed the video which did it from the front. I would probable do it from the front if I had to do it again.
Oh, BTW, it works great now! It only rumbles when I'm actually drying my sneakers!