i have an 06 plate S-Type 2.7TD manual.  it has a slight whirring vibration noticeable at motorway speeds which sounds like a wheel bearing.  However, every now and again when travelling at 70ish a vibration will start from the rear, and it grows in severity becoming quite loud and violent, a bit like hard braking on very warped discs, or a loose wheel, although it's not coming through the front or the steering.  I'm forced to slow down because it's very disconcerting.  It'll last a few minutes before dying away.  It's happened to me a couple of times, the first time was immediately after an MOT and shortly after I'd just had the rear pads replaced.  I had an issue on the way home from the test centre (who hadn't fitted the pads) with the rear n/s brake binding up.  Any thoughts?

Happened again this morning at 70mph, sounds like a chinook hovering overhead and feels like something hitting on each revolution, and is like driving over those speed strips you get on the approach to some roundabouts.  This time only about 5 minutes into my journey but again only lasted a couple of minutes.  Will try and get the garage to lift it today I think.

 Have a whine on certain points of holding the accelerator. I think mine may b the prop shaft centre bearing, could be the same as yours maybe? 

I'd get the rear wheels off the ground and work inwards, checking everything from warped discs, wheel bearing play, bushes, loose/leaking shocks and axle end float. I'd also get someone to drive behind me to see if either of the rear wheels are bouncing slightly at that speed (failing shock absorber?). You might also want to get the wheel alignment checked.

Good luck!