Warn produced the All-Range overdrive for the Model 18 transfer case. It bolts up to the Power Take Off opening on the back of the transfer case. It sure helps slow the engine down at higher vehicle speeds. However, don't expect that you will gain much top speed if using the original flathead 4 cylinder engine. What happens is that with the slower engine RPM, the engine runs out of steam against wind resistance to maintain freeway speeds. And if you drop back out of overdrive for more power, the engine RPM rises to a scary level. With or without the overdrive, the top speed of the Jeep is 60 mph at best, and that really is working the engine hard. The overdrive really shines in the 40 to 50 mph range but above that the engine power will be lacking. At higher speeds you may have trouble making moderate grades in overdrive.
Rick Grover's website has all the information you will need to know about the Warn overdrives:
He also has scanned images of the original installation instructions. Please note these instructions show the early type of Warn overdrive which had the shift rod on the front of the overdrive unit. Later models had the shift rod exit from the rear. Installation is identical except for the shift linkage.
Installation of the overdrive unit itself is pretty straightforward, as explained in the installation instructions linked above. Just pay attention when fitting the locking washer and retainer clip that secures the planetary assembly to the transmission shaft. Without having seen the disassembled pieces, it can also be confusing if you are removing an overdrive from another transfer case for your own vehicle.
Fitting the shift linkage is a bit more time consuming. The OD shift linkage passes through the tight spot between the transmission housing and the transfer case shifters. Expect to spend some trial and error time shimming it sideways for free travel.
The sheet metal shroud around the transmission shifter will also have to be trimmed. My shroud was in good shape and I was reluctant to cut it. I purchased a rust-damaged shroud and set aside my good one. The lower edges of the rusty shroud needed new metal in places. After welding in new metal, I did not redrill one of the bolt holes as shown below. After I figure out some sort of boot to cover all three levers, that one bolt hole would get in the way. Normally I wouldn't bother filling in that hole but since I was replacing that area anyway, I saw no reason to redrill it:
Here is a view from the passenger side:
I haven't quite figured out what to do for a boot to cover all three shift levers. Warn supplied a flat rubber sheet with their overdrives. It reused the Willys dual boot for the transfer case shifters, with a slot for the overdrive lever. I'd have to bend up the new mounting ring to fit the transmission shroud. I'm not sure that this boot would have ever worked very well. The mounting ring has extra holes, apparently for use on trucks and wagons in addition to the CJ series. If you decide to duplicate one, only drill the holes that you need:
Here is a scan of a rare NOS label for the dashboard: