The way the Jag rear is designed is kind of funky in that it uses the half-shafts as the upper control arms. And I would venture to guess that the stress that we put on our Cobras is even harder than the average Jag driver. The bearings and seals can end up taking a pounding. At a minimum, Jag rear owners (and really outboard rear owners as well) should change the lube in a timely fashion. See this Coventry West tech tip:
Dealing with your Jaguar Differential