So one option would be to use the Rock Box DIY kit and the LoMax 4:1 gears, which puts you at $2100 (as opposed to finding a used NP241, which are selling for nearly the same price). You'd also have to get at least a new rear driveshaft because of the length difference - I'm not sure if the front would change. I'm also not sure what else you'd need to buy to put this all together - it says the Rock Box DIY comes with complete shifting assembly... Great option for standard wheelbase TJs, brand new 4:1 t-case, fairly straightforward setup.
Another option would be to source a second 231 ($150-200), get the Froehlich adapter ($475), 6-pinion planet gear ($150), short SYE kit ($450), (optional 231 rebuild kit - $250), and a pair of cable shifters ($340 for 2). Beef up your current 231 with the 6-pinion, rebuild kit, and SYE; weld up (not sure how much someone would charge for this for those of us without welding capabilities). You might be able to get better prices on some of these things - I just did some quick google searches - but all of this adds up to about $1800. Or, applying the principle of Jeep Parts Cost Justification, if you already own (or were already planning to buy) an SYE, one of the cable shifters, etc., this build could be more like $1200. You would definitely need both new driveshafts as the entire t-case is moving back - front will need to be longer and rear will need to be shorter. What else am I missing?