Fifth Horseman

I know it sounds obvious, but make sure the nut for the flange is tight before doing anything else - I had a similar vibration on a D2 and it turned out to be the flange nut loose. If the nut is not loose then with the amount of play you describe then the bearing needs replacement. After being stressed by a seized propshaft it is probably best to replace the bearing anyway. The flange should only need replacing if it is warped or if the surface the seal runs on is damaged.

They generally only do that when there is no more fluid to leak out. To check for play in the steering box, disconnect the link from the drop arm. There should be no play with the steering wheel in the straight ahed position (this is easier done with two people - one checking for play and the other holding the steering wheel). When you turn the steering wheel from lock to lock, does it move smoothly? - if not then it is likely that the bearings in the box are trashed (new box time)

Don't think so, a quick search shows the following Front to KA930455 FRC4320 from LA930456 FTC3154 Rear 90 to KA930455 FRC3132/FRC8540 90 from LA0930456 FTC3188 110/130 LA930456-WA769310 FTC1740 110/130 from WA769311 FTC3188

I'd go Polybush or similar. I had the OEM rear shock bushes go after 4 years ('07 Defender), noticed it before MOT time as a knocking from the rear. Both top and bottom bushes had worn. Fitting was a simple remove and replace - top shock mount will either press out or may need to be cut into sections, be careful not to damage the shock eye if you do this. Its important to replace the lower shock bush if there is wear as in time the shock end pin can wear the mounting hole of the axle bracket.

If you haven't already, replace the air filter as soon as possible. If the filter becomes loaded with enough oil the engine will start to suck this into the engine in the form of a mist, which can in extreme cases be enough for the engine to run on. Symptoms can be the engine seeming to run better than usual and taking longer than usual to stop when the ignition is switched off. There have been cases where the engine continues to run and accelerate with the ignition switched off, stopping only when stalled by putting the vehicle in gear and dumping the clutch (brakes on) or stopping in a terminal way when the motor exceeds its design speed and internal parts get scared of the dark.....

If the warning light goes out just above idle then comes on faintly at higher revs then it could be a diode in the alternator regulator thats gone short circuit. A stated first thing to check is the drive belt and also the connections.

Hardly "many thousands of rpm", at 60mph the wheels are turning at about 670rpm. With the propshaft connected the diff will be turning and so will the remaining driveshaft so there would be minimal bad effects on the diff as stated by the OP. If the propshaft is disconnected then with the vehicle moving then the diff will be driven via the planet gears - not good. The vibration could be from either worn propshaft u/j's or wear in the sliding section or just as likely a result of driving with the centre diff locked.