1990 and up 4.0-liter Rangers are always equipped with the 8.8-inch 28-spline rear axle, unless it's an FX4.
Rear axle codes can be found on the Safety Standard Certification label attached to the left driver's side door lock post. The code appears as a number or letter/number combination in the 'Axle' column of the label.
The ARB locker is discontinued for the 28 spline 8.8".
The Eaton E-locker is available, similar price but without the cost of air lines and a compressor.
The Auburn Ected is not a real locker. It is a limited slip that "tightens" up using electromagnets. It does not fully lock.
You can also get an automatic locker (lock right) and install the lunch box locker in an open carrier, and detroit may offer a full carrier replacement automatic locker for the 28 spline.