If that poor Frontier is an auto I just feel sorry for it.. I'm guessing the "towing package" on the truck consists of a hitch? OR does it also have a tranny cooler, Class IV hitch, power steering cooler, etc?
I'd venture to say any kind of grade and you're going to be better off driving the XJ. Also PLEASE do not tow with a U-haul trailer with surge brakes, man that's just scary...
If you must tow with such a small truck get yourself a nice trailer w/ brakes on both axles and a good controler.
Having towed w/ my old 01 WJ Limited and my cousin's 06' Quad cab Chevrolet and now my 05' 2500 cummins.. it's just amazing the difference a 6spd diesel makes. I set the cruise on 65-70mph and just let it go in 6th...
For comparison I've ridden with my cousin from Winston Salem -> Mountain City and that's a pretty large uphill battle and his 1/2ton Chevrolet pulled it but wasn't too happy.... pulled the same thing w/ my truck and well it's night/day difference..
So basically could the Nissan "theoretically" pull the XJ around, yes... should it, no...