First of all, take a look at these sites:
http://www.1010tires.com/TireSizeCalculator.asphttp://ejelta.com/tiresize/Very useful (especially the first one) to tell you what will fit on what rim. I checked that 1010 site and they state a 215/75-15 will safely fit the stock wheels (which are 5.5" wide). Unfortunately they recommend a 6-8 inch wide wheel for a 225/75-15 tires, and a 6-8.5 inch wide wheel for a 235/75-15 tire. Although it would probably still mount up properly, I bet would would pop the wheel off the rim if you really start pushing hard off road.
I am no expert. Heck, two days ago I was asking the same question on this forum (see the posting "HOW BIG CAN YOU GO").
But in my opinion, rather than trying to stuff big wheels (on stock rims) into a tracker without any lift, you should look at a different option. The stock tires (mine came with Uniroyal Tiger Paw) are total crap. Look at different tire manufactrers offering all-terrain tires in a size that is close to (or slightly larger) than your stock size. Its not just about stuffing a big tire on your truck...if the tire design sucks, it doesn't matter how big it is.
Bridgestone and many other manufacturers make reasonably good off-road, all-terrain tires (not mud tires) that will be slightly larger, fit your stock rim, won't rub the body, won't require a lift kit, but will give your truck much better traction.
Oh yeah, and stuffing a locker in the rear diff wouldn't hurt either.