Hi Excaliber,
Re: the Jag rears, as far as I'm aware the diffs are all Dana 44s (8-3/4") with slightly larger diameter ring gear bolts.
The early E-Types did have less stout stub axle bearings, but the later bearings can be used.
Otherwise they're pretty much the same all the way up to the nineties' version of the XJS.
The big differences seem to be in the gear ratios and whether or not they are limited slip ("Powerlok"). By the way there are apparently two different Powerloks - one for the higher numerical ratios and one for the lower, e.g. 2.88 and nearby. I forget exactly where the break is - 3.08???
The other big difference is in the widths of the axles & wishbones. The E-Type is the narrowest, the 3.8s/420 are next narrowest and the XJ6/12/S are the widest. The first two may (depending on the kit!) be used as-is, the others almost always require narrowing.
Hope this illuminates more than it obfuscates
Regards,
Tom