The rear diff lock electrical system is highly exposed on the new 2021 Dmax, which I assume will be the same on the BT-50. The diff lock electrical is all located on the forward facing side of the rear diff. Probably not much of an issue if you never go off-road and use the diff lock. Major issue if you do go off-road and need to use diff lock.
Electronic diff locks need electrical systems to function, and whilst this is a design element of the GKN differential that Isuzu use, I would expect Isuzu to provide some protection for the system to ensure its fit for purpose as a 4WD vehicle, so the important system is not damage and rendered useless when you need it most. Thanks to @Jake Minogue on Youtube for highlighting the manufacturer of the system.
See the issue for yourself, two pieces, one hanging off and very susceptible. The top one susceptible to branches and such.
The GMF cover completely covers the two parts with little room for anything to get in and do damage. GMF have an optional piece to cover some of the wiring too. The IronClad cover leaves the diff lock system exposed at the base, providing some cover, but not fully covering the two pieces.
GMF is my recommendation, see for yourself.
The IronClad cover leaves the lower components exposed.
Both provide cover, so something is better than nothing, however, if you’re an off-road driver, the GMF is going to provide the best protection for you, hopefully you can see for yourself, now showing the two covers for comparison.