Jeep Rubicon has the same problem, I just learned that there is a cure for it through an electrical bypass surgery
which tricks the onboard computer into thinking the vehicle is engaged in 4LO when it is still in 4HI, and ...voila, the diff-lock will engage.
I am still investigating on how this can be done on the H3, will let you know of any findings. on the Rubicon, I heard it's a simple wire bypass in the dashboard.
DuneH3