Holy crap! I literally came on here to post about dang near the exact problem. Above 65 mph is when it starts to get that low frequency vibration and "whoomp, whoomp, whoomp". 65+mph the vibration starts is higher (the oscillation gets faster) and the vibration gets a bit worse. Happened with my slant six /904/8.25, now it happens with my 360/904/8.25. Common denominator is the 8.25. Before we go in with a dial indicator, we pulled the inspection cover off and found the spider gears with wear and they move in an up and down fashion. My ring and pinion look fine. No shards of metal. Do your spider gears do the same? I have a video of it below.
Apologies I'm not trying to hijack your thread. I'm hoping I can provide anything you may not have looked at and vice versa.