Some time around 1962, Warn, the hub and winch people, came up with a compact design for an overdrive that bolted to the PTO output of the model 18 transfer case. Actually, I believe it was originally developed by a couple of brothers, Chet and Norm Thompson, who sold the idea to Belleview Manufacturing, which, in turn, was bought out by Warn. The first ones offered you a choice of either a 20 percent or a 30 percent overdrive ratio and could be used in four-wheel drive. It then later went to a 25 percent overdrive, which actually did a great job of splitting each gear in the tranny. The Warn unit was around until sales slowed, and in 1988 it was taken off the market. In 1991 Advance Adapters purchased the remaining inventory, blueprints, tooling, and renamed it the Saturn overdrive, which is still being made today.
There were a couple of imitators made by Husky and Dual-A-Matic that had limited success. And while they looked similar, parts were not interchangeable.
I remember spending $225 for the Warn overdrive in 1966. I sold the very same overdrive a couple of years ago for $800. So good was this overdrive that Jeep made it a factory option from 1964-'70. Even the military used it in its version of the FC-170 diesel-powered crew cab trucks.
For a while I ran both the Rancho and Warn overdrives in conjunction with each other. It made me feel like a trucker with all those gears to shift. Seems that I broke a few teeth off the Warn overdrive a couple time. Perhaps it had something to do with power-shifting it from direct to overdrive when drag racing.
The Saturn overdrive and rebuilt versions of the Warn are still popular with those who keep horsepower levels below 200 and retain the Spicer 18 transfer case.
The Rancho overdrive came out around 1958 in a joint development between Clarence Shook and Basil Smith. It consisted of a special adapter plate and a '46-and-later Studebaker overdrive unit. | 
Studebaker overdrive was the unit that was the basis for the Rancho Overdrive for Jeeps. | 
The Sierra overdrive was developed for the Dana 20 transfer case in the early 1980s. |
Unfortunately, neither the Warn or Studebaker version of the Borg Warner overdrives would work with the new straight-through design of the Dana 20 transfer case that came out in 1972. But factory axle ratios had become more manageable by this time, and the four-speed T-98/T-18 was an option. Even automatic transmissions in the form of the super-strong GM TH-400 were available.
In the early 1980s a small machine shop produced a few prototypes of the Sierra overdrive for the Dana 20. I got involved in the development, and after some lubrication problems were solved, it proved to be a strong unit. Unfortunately, lack of development capital contributed to its demise.
Around 1969 Warn built an overdrive very similar to the Ranger overdrive offered today by Advance Adapters. Warn referred to it as an auxiliary two-speed transmission. This went between the T-18 transmission and the bellhousing. It was short-lived, and I only ever remember seeing one of them.
The Spicer 18 transfer case first appeared in the MBs and GPWs of World War II and was in production up to 1972 and came in 13 different versions over the years. Compact and quite strong, it is still being used by off-roaders today, and, in fact, there are several companies that specialize in custom gearing for them. One of the drawbacks of the intermediate gear design is that it rode on a shaft supported by a series of small needle bearings. This shaft never received sufficient lubrication, and both it and the bearings would wear and become quite noisy. It's also thought by some that this shaft would actually flex under load, contributing to the wear and noise. Spicer, in an effort to solve the problem, increased the size from 3/4-inch (as used in the MB versions) up to 1 1/8-inch in the early civilians and finally ended production with 1 1/4 inch shaft. I believe it was Archer Brothers Jeep in Hayward, California, that came up with a way to eliminate this shaft and troublesome needle bearings. The company machined the ends of the intermediate gear to accept tapered Timken bearings. However, it was difficult to always get the correct bearing adjustment. Now it seems that with superior lubricants, the way to go is with the original design.
Believe it or not, there actually was a three-speed version of Spicer 18 built back in the 1960s. Thing is, it wasn't built by Spicer. It was made from plate steel that was welded together and then heat-treated to relieve the stress before being machined. It had to be about 1962 or 1963, and I was for some reason or another in Buschert's Machine Shop in Hemet, California. Harry Buschert was showing a buddy and I this prototype that he had developed. I don't think that there were over a half dozen ever made, and each one was a bit different. I wouldn't swear to it, but I believe it had a lower low range than the normal 2.46 gearing, a 1:1 high range, and an overdrive range. Harry was on the right track and way ahead of his time when he built these. Little could he imagine that some day a three-speed and a four-speed transfer case would be in production.
Today we have a wide range of transmissions - from standards to automatics - available, including six-speeds with built-in overdrives. Plus, we have adapters available to put just about any transmission to any transfer case your mind can imagine, as well as separate overdrive units and gearboxes that fit either in front of or in between the trans and transfer case. The old standbys - the Spicer 18s, Dana 20s, and Dana 300s - are being modified with lower gears and stronger parts to survive four times the horsepower and torque they were originally designed for. Transfer cases that were originally designed for much larger vehicles, not even carrying the Jeep nameplate, are being successfully used. Best of all, there are at least three custom-built transfer cases that can withstand all the abuse you can deliver. Sure beats the heck out of rebuilding your T-90 several times a year and new bearings for your Spicer 18 when you could no longer stand the gear whine.