Not a lot you can do to improve an XJ if you still want good offroad handling but here's what I'd do if there was no need for offorad use.
Sell it, buy the BMW
Failing that,
Swaybars front and rear
Lower it
Low profile tyres 18" rims
Firmer shocks and coils but dont stiffen up the rear to much
That's about it, if money was no object IFS the front and IRS the rear, those solid axles dont help in the handling department.
Cheers
Steve