While it is difficult to give an accurate on line diagnosis, I would suggest that rather than immediately suspecting the rear diff,
You may find is more likely to be either a UJ on the drive shaft, a partial handbrake seizure on a caliper, or a wheel bearing, the actual rear diffs are normally quite reliable and usually either work or internally explode,
As for a diff specialist, you are better replacing it with a (known to work)second hand one as the cost of rebuilding/revising is prohibitive.