I have the posi-lok and it works pretty good.If your good at fabricating it might not be too hard to make one.I think the hardest thing would be making a housing to fit on the axle,all the posi-lok does is reuse your shift fork with a cable and new housing.It was easy to install,messing with the c-clips that hold the fork on was about the hardest part,took me an hour or less to install.If you look Quadratec or J.C.Whitney's site there is a pretty good picture of one.
Transplanted '99 4.0, AX-15, NP231
Red Jeep club #8
97 ZJ Laredo
4.0, 42re, NP242
Was my daily driver, now son's college wheels.
I highly recommend the posi-lok as well. Put one on a few months ago after many problems with the vacuum actuated system.
Took me about an hour, and wasn't too much trouble. If you're down there, I would recommend replacing the shift fork as well. Another $10-15, but it's worth it. Haven't had any 4x4 problems since. Good luck.
Did this conversion tonight, what a breeze (for me anyways). Used a 3/16 air fitting to mount the cable through the shifter housing, tapped it, threaded it in, fed the plastic wrapped cable in and tightened up. drilled 2 holes in the shift fork and threaded the cable through and back, cut it and bent it back against the fork (that was the hardest part IMO), zip tied the cable to the axle and threaded it through the engine bay. I already had the cable mounted in the cab for my throttle, so I just swapped the purpose of it.
The air fitting seals around the cable tightly enough as not to leak, and for me was free, but if bought would only run like $1-$2, the cable originally cost me $25, and well about an hour of my time.