I have had 2 gearbox's that rumble when i put the car into 5th gear, i heard this was a bearing at the back of the box, is the re anyway of replaceing it?

Yes, either the 5th gear bearing or the gear itself, but almost certainly the bearing.

Yes, it can be replaced, best to get hold of an older haynes manual, they have a good guide to stripping and rebuilding the type 9 'ox.