Very nice XJ, and it will look way better now with that meaty tire on there. As far as 4wp goes, they are just watching they're A$$ for liability reasons. If you oversize your tires and it rubs all over the place or rips part of your bumper off, or the sidewall gets cut on the stamped control arm they don't wanna be liable, especially in law suit happy america against a big corporation. (i've seen it happen on some really thrashed stock arms with horrible backspace, i know your running steel wheels so your b.s is good) What they should have done is made you sign a disclaimer that you wont hold them liable. Like you said most guys back in your town are just hardcore muddin type guys.
If you lived in SoCal and came to the laguna hills store where 4 of the employees actually own and wheel they're XJ's on real trails, you would be treated quite nice. Glad you got the tires you wanted, and sorry you had a bad experience at 4wp. Just keep on wheeling and have fun out there.