Yep as Steve mentioned... there no
real chassis.. its monocoque as u have discovered... but dont be fooled its mighty strong... !
Yep all or most sliders mounted to a XJ are done via the lower pinch seam on the lower door frame rail.. !! also most designs incorporate mostly 2 x support bracket sections, bolted directly to the built in chassis rail ..
this is mine in the final stages prior to painting.. you can see the chassis supports and pinch seam rail tacked on to the main bar...
Here is a pic of AJ's Offroadarmour sliders, again you can see how they are mounted, just like mine !
cheers