I thought I would throw in my view on the age old question of shocks. My XJS had a set of bilsteins in it. They were black so not sure if they are OEM. Anyway, I went to a new set of Boge... It feels like I am driving a bowl of soup. Rides great but steering response lags and its quite soft. I think Im going back to the bilsteins. They were much better.

 

Yes, it is often necessary to retune a chassis that has had its weight balance and center of gravity altered by the installation of a completely different drivetrain than it was originally designed for.

 

One other footnote on the Boges: there are different valvings for the various Jaguar models, and an aftermarket supplier might be selling one specification as a "fits-all". You might have installed the wrong application from the right manufacturer.