Changed out both rear wheel bearings, no change, still roaring like 747, goes away under acceleration, how hard is it to rebuild the rear diff?

 

I have never rebuilt a differential, so I don't know the difficulty of it. However, I think the service manual gives a pretty detailed layout of how to do it.

 

Setting the backlash and gear pattern is the hardest part. If you are just rebuilding it then you can keep the same parts in order and it is a little easier. The Rover is great in that the diff is a center section drop out and you do not need to remove the axles. You could have a bad pinion bearing or something like that. You may try a wrecking yard or Roverlandparts to get a replacement, it may be easier.