I would like to put a lower gear ratio in my rear and preferably a posi like 3:73 Any suggestions....

 

Do an "advance search" for limited slip differential or LSD. I am not aware of a posi differential.

 

Did the guys who swapped your LS motor keep the factory diff? I think Jag Specialities uses a GKN 15HU diff from the 90-97 XJ6 and XJ12, a lot stronger than the crappy 14HU in the V8 cars. The 15HU diff itself will drop right into the IRS assembly of the XJ, dunno about the XK, you’ll need to get the back triangle plate because of the offset hole for the pinion flange. Main thing is gonna be making a custom driveshaft with a UJ on the transmission side.

3.77 factory Powr-Lok diffs are out there but 3.58s are the most common. Internals are pretty much all Dana 44 stuff and the 3.93+ carrier should fit into an open diff if you can find a shop willing to do the swap. I’d assume your GM TCM can accommodate a change in rear end ratio.

 

I would like to swap mine out for a limited slip as well... I am considering the Quaife differential which is limited slip. Just need to wait awhile as I dropped a boat-load of cash on my LS swap this year...

 

3.58 P/L and a custom driveshaft is much stronger and can be done for less. Get rid of the junky 8” diff all together, you will blow up your $1200 quaife and you won’t be happy when it does