Well, only illegal until you get it engineered.
I was at Opposite Lock the other day, (primarily looking for headlight replacements), & asked about lockers. Interestingly, they said that while most people think of rear lockers first, front ones are more useful for pulling you out of trouble, where rears just tend to bog you down more when used alone, (ie. without a front).
Off topic, he also mentioned, when speaking of engine replacements, that there is someone local who has put a Toureg V10 diesel into his JK... Apparently it didn't take much engine mount modification, & he described the power & torque from it as 'scary' to drive. Fun, but scary...
Haven't voted yet, but I already have an ARB onboard compressor, I'd tend to go with one of them, probably. From Just Diffs or similar, where it is cheaper to buy & get it shipped, than to buy locally...