Sure enough that wheel had quite a wobble. These wheels are quite new, so in shock I took the wheel down to the local tyre place and had them put it on the balancer to check it out. A relief that the wheel was ok, but thinking that must mean I have a bent axle. The diff was gonna get reconditioned anyway, so I took the whole lot in to the diff place, and sure enough they had to straighten that axle.
Now I thought everything would be good, but after reinstalling the diff, I put it in gear on the axle stands again, and the wobble was still there.

At this stage I called the diff place again to see if they had any suggestions, and they suggested maybe the drum wasnt seating properly and to file the centre out a bit. Tried that with no luck.
Im really stumped as to what else it could be.
Any ideas anyone?