i have 2 GSDs. both female, one 95lbs and the other is 65lbs.

before i got the white one, the other would fit, but was tight for her to turn around, and she likes to do that a lot.
i always kept the back seat down just for her comfort, but even with the tj, i had the back seat removed. they both could fit there, tight, but comfortable enough for them both.

i'm sure you would be just fine with the xj as you as i'm sure you won't have the seat up all the time.
i don't think the zj has all that much more room anyways, so go with whatever jeep you like more..... hopefully the xj. lol.

sorry, i have no pics of the dogs in the xj as we now have a minivan a.k.a. dog transporter.

 

My XJ is the dog hauler, My wife doesn't want her car filled with dog hair. And the leather seats in the XJ are easier to clean. Now if I just get them to stop leaving their nose marks all over the windshield, and windows.