The early XJs with the Command-Trac transfer cases had a vacuum disconnect in the passenger side axle shaft of the front axle. In 2WD this was intended to reduce drag and improve fuel economy.
However, the shift fork was prone to fatigue, and I guess DaimlerChrysler finally decided the minute improvement in mileage wasn't worth the expense and complexity, so they stopped using it in the early 90s. The YJ (leaf spring/rectangular headlight) Wranglers had the same setup, and it was dropped when the TJ (round headlight/coil spring) Wrangler came out.
High pinion means that the pinion gear axis is above the axle and ring gear axis of rotation.