Play nice - he said he was a noob!
John - you're not going to be able to lift the body on the XJ - it's a unibody, and the "frame elements" are welded to the body.
I've fit 30" tyres under a stocker (although earlier body style - I'm not sure about the later that you have) with some rub at full lock and a little loss of articulation. 30's and 31's will fit neatly under 3" of lift, and 1.5-2" of "budget boost" (blocks and coil spacers, essentially) should fit 30's with enough room for articulation - although you're still going to rub at full lock (that's not a function of lift, that's a function of wheel offset/backspacing. To fit a wider tyre without rubbing, you'll need less backspacing to set the wheel out from the body some more.
30x9.5-15 is a common enough size, and you should be able to score a decent tyre for a decent price.
Youre 235-75/15s are nominally [235m/m ~= 9.25". 9.25" * 75% ~= 6.9". 6.9"x2 + 15 = 28.8) twenty-nine inches tall, so you'd be gaining about a half-inch of under-axle clearance (recall that you'd only get half of the increase in diameter - or just the increase in radius - when you go to a larger tyre. Going from a 29" tyre to a 31", you'd net one inch of ground clearance under the axle housing.)
Fortunately, powerline easements and fire roads are fairly decent - I've done them in stockers with stock tyres without any trouble. Just pick an aggressive tread (I believe BFG makes their AT/KO in our stock size, and I've found that to be a decent all-purpose treat pattern. Air down just a touch to increase ground contact and therefore traction - 5-10#, tops. Air up before you get back on the pavement.)
The formula I used to figure tyre height for you works like this:
Tyre size is given as "tread width(m/m)/aspect ratio(%)-rim diameter(in.)"
Convert the tread width to inches (in = m/m / 25.4)
Multiply the tread width by the aspect ratio (75% = .75, in the example here.)
This gives sidewall height. Double this number (two sidewalls) and add to the rim diameter. This gives the overall nominal diameter of the tyre (you can, for most manufacturers, also look up the actual rolling diameter of the tyre - it will be slightly lower than the calculated nominal diameter, owing to the tread patch being flattened at the point of contact with the ground.) However, the calculated nominal figure is close enough for most purposes.
Light Truck tyres are still not sized using the "metric" system given above - they're technically oversized, and are sized as "nominal diameter(in) x tread width (in) - rim diameter (in). No calculations are necessary to get the nominal diameter, but if you're after accuracy you should look it up on the manufacturer's site (for instance, the BFG 31x10.5-15LT AT/KO has an actual rolling diameter of 30.8". This small difference can be important when, say, selecting a speedo gear to complement lift and gearing in addition to new tyres...)
I mentioned "backspacing" and "offset" - here they are:
Backspacing is the vertical distance between the wheel's mounting surface and the inboard bead flange. Typically, you lay a straightedge of some sort across the inboard flange, and then measure the distance between that as the wheel's mounting surface using a rule. Backspacing is always positive.
Offset is the vertical distance between the wheel's mounting surface and the theoretical centre of the wheel. Take a 15x7 wheel (15" tyre inside diameter, 7" between bead flanges.) A "zero offset" wheel would have the mounting surface exactly 3.5" from the inboard bead flange - if it's less than that, it's positive offset; more is negative (I believe. I may have those reversed - I usually think in terms of backspacing instead.)
