Well, it's one of those dense composite plastics. The only reason it was of concern is because in my older ARB, the plastic cradle has a crack on it and appeared to interfere lightly with a spider gear. The crack doesn't compromise anything, as it's not in a critical location, but it's just one of those things that shouldn't occur (may have been from a prior disassembly by a previous owner?). The TJM engages the locking gear in the same manner as the old ARBs, but the TJM cradle is comprised entirely of steel parts. The new style omits the cradle all together, as all the moving parts have moved to the opposite side of the locker; it's a major improvement over the older style, in my opinion.
New bearings are required and are not included with the ARB. The TJM also requires new bearings, but they are included. OEM bearings will not fit either locker. In the ARB Application Guide, lockers RD134, RD135, and RD136 (all H233B lockers, 31 or 33 spline) have a design note indicating the requirement and bearing part numbers: Timken 32010X and ARB 160116 (I think the ARB part is the Timken...yet 2x the cost). That bearing is also apparently used on ARB lockers for some Toyotas. Also, it appears that the Nachi E32010J bearings that came with my TJM have the same specs as the Timken: 50mm ID x 80mm OD x 20mm width (20mm cone width x 15.5mm cup width). The Timken's appear to be the cheapest and most-available option of the three.