Like you will read many times here, nearly every XJ is different. On my '89 I can fit 31"s stock, fenders trimmed, and on my father in-laws '95 he has 30"s with plenty of clearance with no trimming. I guess it all depends on how your suspension has worn or sagged, and truly how tall is was to begin with. Before I trimmed my fenders, I measured from the center cap of wheel(center of hub) to bottom of fender flare, and it came out to 1.5" and .75" higher in front and rear respectively from the "stock" measurements that have been posted in these forums.
Just find out how large your tires are (225/75r15s are 28.3 inches tall) and measure the clearance to the closest point on your flares (more than likely the frontmost and rearmost parts of the flare) and decide how close to the flare you are willing to go, and depending on how you use it, pick your tire size.
Remember, if you have 2 inches of clearance to the area of least clearance and you are willing to only have 1 inch of clearance (dont know why, but this is just an example

) then you can run a 30" tire for what you are looking for.
If this confuses you in any way, a 2" increase in tire height only increases section height by 1" (section height is the area from the top of the tread to the edge of the wheel) and thus a 2" taller tire only nets you 1" loss in clearance.
Hope this helps you decide
Jeff