"I can't recall where exactly, but I've read at least a couple of things saying this was it exactly — Rover had trouble with drivetrain vibrations in the P6 and UJ phasing was part of the solution."
That is exactly it....Hence the harmonic damper and steel plate on the torque tube of the diff. Also the tailshaft doesn't run straight back to the diff; it's fractionally offset to one side. I've half a dozen rebuilt P6B tailshafts (ex Flynn Auction) and every one has paralell universals as that is the standard build. Just like the drive shafts on P6. If your car has had its shaft rebuilt that may be your problem. Note that the (rear) suspension/drivetrain can also cause vibrations due to how it works so you need good mounting points etc. and your wheels and tyres need dynamic balancing. You may have just thrown a weight or the available weight was a little bit light or heavy (as that doesn't show up at the speeds commercial balancers run at).
On my LT77 install I checked the flange angles with the engine/gearbox and diff set where I thought it would finish up, and the angle between them was < 2d...BUT the final rear gearbox mount install almost certainly changed that a little. Proof being in the pudding, so far up to ~ 110kph I have not experienced ANY vibration, just a fine inaudible vibration can be felt on the gear lever. All the diff mounting rubbers are NEW - the nearside diff bracket rubber was well shot and sagging, while the offside looked as good as new.