I own at 98 (12v) and have a vac operated front axle. I got an electricially (12V) vac switch from my parts house. It's a Ford (sorry) egr switch for a van. I put it in line between the t-case and the front vac housing with a switch on the dash. I can put it in 4 low and not turn on the switch and I have better than 7 to 1 rear gears without front drive.
A month ago while driving up the back side to Big Bear Lake Calif. with a 11' camper, car trailer and Bronco on it I was able to shift into 2 low and pulled the hill like it wasn't there. As there were/are a lot of switch backs speed wasn't a question. Also the trans NEVER heated up above 180. Egt's didn't suffer either.
A month ago while driving up the back side to Big Bear Lake Calif. with a 11' camper, car trailer and Bronco on it I was able to shift into 2 low and pulled the hill like it wasn't there. As there were/are a lot of switch backs speed wasn't a question. Also the trans NEVER heated up above 180. Egt's didn't suffer either.