X2 on the ARB. I have had the one in my "90 going on about 7yrs now and no leaks.

The one in my '91 leaked after about 6 engages on the trail, but that was because I didn't get the new style diff housing fitting tightened correctly, to little and it backs out/too much and you tear the O-ring. The leaks my friends have had have also been from either damaging the locker(backed into a rock and dented the housing), running the air line too close to the exhaust manifold, or a loose fitting, all operator error.

You can swap in a Tacoma/'96 newer 4-Runner e-locker with some modification to the non-Taco axle housing, or just swap the entire rear end in. The costs are about the same for a new E-locker/geared diff, and used prices are about the same as well. For the ARB, your buddy can either buy teh ARB compressor, or make his own using a small air tank, and MV50 Superflow(or other named) comp, an ARB pressure switch, and some air line fittings, and save about $100, less if he uses a used comp. I use an old air ride suspension compressor for my ARB, and a small air tank I fab'd up.