yep sounds like the differential has gone from 4th to fith so you may need to get that sorted or prob cheaper jus to get a second hand box
'Differential has gone from 4th to fith'?

You only get one differential per pair of wheels that drive the vehicle. The bearings on the differential have are likely to have worn.
Yes it's cheaper to get a second hand box, but you get what you pay for and what do you actually want? Your box could have been second hand if you sold it to someone, and look what's happened to it.
Get a recon at the very least.