Tech information furnished by posters on the IH8MUD.com forum...
[The purpose of this modification is to shift your 80-series into low range and NOT have the center diff lock activated.]
Remove the dead pedal and the left front sill plate. In the left kick panel you will find a Silver metal box. That is the ABS ECU. Remove it, and behind you will find a black plastic relay about the size of a cigarette pack. It says "transmission control relay" on it. That's what you are after. The EWD calls it the "center diff lock relay", same thing. Unplug the harness from it and take a terminal pick and remove the black wire with blue tracer from pin#7. Insulate the bare terminal to suit and then put everything else back where you found it. If you have the low 4 position swith at the transfer case unplugged, crawl under there and reconnect that as well.
What you have just accomplished is you have interrupted the signal from the transfer case that tells the lock ECU that you went to low range but you are still sending a signal to the transmission ECU telling it that you ARE in low range. Therefore the transmission will shift in the correct low range pattern. In addition you now have true manual control over center diff locking. It only locks when you tell it to without regard to transfer range selection.
In order to do that you need a center diff lock switch installed in the dash. Very easy to do. 10 minutes tops to do. You can get one from sleeoffroad.com They are about $50.
The Center Diff Lock switch is only the first step to achieving an unlocked center diff in low range. You will then have to either remove a plug directly from the t-case, or do some modifications to your wiring. Both options have thier ups and downs.
It is not as easy as just installing a switch.
BTW, I have installed the switch and 10 minutes is a good estimate if you don't plan on removing any screws and just ripping the dash panel off, but plan on 20-30 minutes to do it right and make it nice.
I have not yet done the mod to run without the center diff locked in low. I haven't needed it, so haven't wasted time on it. If ever I do, I will do the wiring mods and leave the connector on the t-case so I don't get muddy goop in my connections.
Can anyone describe how the shift pattern changes from low to high range? Does it shift up/down sooner/later?
I got the switch in, and unplugged the plug a few years ago.
The transmission shift points move up in low range, it also downshifts earlier. It is difficult to verbalize but very noticeable.
You can check it out like this:
Shift into low range and transmission in drive. Accererate moderately. The trans will go 1,2,3,od quite rapidly.
Stop and crawl underneath, plug the low 4 position switch back in and repeat the test. You will find that the gears hold for much longer, and when you let off the trans downshifts.
Give it a try...................