If you've been following our Tacoma project you know we're working on a 2WD Prerunner with a 3.4L V-6 and auto tranny. We chose this model because we knew we'd be torching off the front suspension to install a solid axle. We also had ideas to use something other than a Tacoma transfer case. The Prerunners are available at a considerable cost savings as compared to a 4WD Tacoma and we knew of a solution for adapting the 2WD auto tranny to accept a transfer case.
Inchworm Rock Walkin' Gear manufactures a billet aluminum adapter plate that replaces the 2WD transmission tailhousing and makes the tranny output look like that of a GM TH350. From there, any number of transfer cases can be adapted.
We narrowed our choices to several configurations we considered. The first was a solution that kept the driveline essentially Toyota. Inchworm offers their Lefty Toyota transfer case conversion that utilizes the internals from the older style Toyota gear drive case to flip the front drive output from the passenger side to the driver side, as found on the Tacoma. Their unique conversion can also be supplemented with a second Toyota gear reduction and combined with low range replacement gears offering deep 11:1 gearing with four forward speed ranges. To build a full Toyota configuration we would also upgrade the output shaft to a chromoly replacement (comes on Lefty) to upgrade that one area of vulnerability in the Toy case, and we would add a twin stick conversion to allow for 2WD mode in any gear range.
Another option that came to mind was the use of an Advance Adapters Atlas II transfer case, or better yet, a 4-speed Atlas that could also provide us with plenty of forward gearing choices and deep gearing as well. In talking to Jim Christiansen at Inchworm, he was able to come up with an adapter scheme that would allow us to mount either Atlas behind the 2WD auto tranny. However, we found that the downside was that this placed the transfer case close to the tail of the transmission. Our desire was to keep the front output of the transfer case as high as possible and the shorter distance to the longer travel solid axle would result in a steep driveshaft angle at droop.
We sourced a transfer case adapter kit from Inchworm Rock Walkin' Gear. The billet aluminum piece on the right mates to the 2WD auto tranny and when combined with an Advance Adapters TH350-to-Toy transfer case adapter (left) starts us on our way to getting 4WD in our Prerunner truck. All the mounting hardware is included along with the female splined coupler to mate the tranny output shaft to the transfer case input shaft. | 
The Inchworm adapter provides the machined holes to accept the stock Toyota speed sensor for the transmission. Once we installed the adapter, we simply plugged it back into the harness. The speedometer function works separately and will be addressed in a future project installment. | 
We started the adaptation by removing the driveshaft flange from the 2WD Prerunner auto tranny. The tailhousing is held in place with six bolts. Once these are removed the rear nose cone comes off after the speedometer drive and speed sensor components have been removed. |
Our final choice combines a bit of each of the above parts lists. We decided to use a 2-speed Atlas II with a Toyota gear drive reduction box adapted in front of the Atlas. This setup will also give us four forward gear ranges combined with the superb durability of the Atlas. The Toy reduction is 2.28:1 and we chose a 3.0:1 Atlas for a low-low range gear ratio of 6.8:1. This should provide us with good gear choices for all wheeling possibilities from easy trails to snail's pace crawling.
The full Toyota setup is reliable and compact and Inchworm's Lefty case makes building a Tacoma setup easy. The cases use Toyota driveshaft yokes and the setup is clockable so you can get the front output tucked up and maximize ground clearance.
The cost of going the Atlas route is a little higher but we gain an essentially unbreakable transfer case and the ability to select front and rear drive outputs independently. The Atlas has proven itself over and over in many hard core rigs. The one issue that we knew we'd have to deal with was fitting the larger Atlas case under (or in) the cab of our truck, and getting it to fit in the confines took a fair bit of fab work. We'll give you a few details in the photos as to how we stuffed it in our Tacoma. We're sure we'll appreciate the stoutness and versatility of the Atlas choice.
With the transfer case installed we'll be looking to continue the project with the front suspension. Check out the next installment on Project Venture Toy to see how we get our Solid Axle D60 mounted to the front end.
The Advance Adapters TH350-to-Toyota reduction box adapter was bolted to the rear of the Prerunner adapter. This casting has an additional transfer case shifter exit on its top for use with forward shift rails that would normally exit through the tail of a Toyota tranny. We used this shifter configuration as it lines up fairly well with the floor plate opening in the cab as you can see here. | 
We're using the front half reduction box from the earlier Toy truck line as our first point of reduction. Advance Adapters offers a beefier replacement casting (right) with side walls twice as thick as the stock unit (left). We used the stock 2.28:1 gearing but lower aftermarket gears require grinding the internal case. The Advance unit comes pre-machined for the larger gearset. | 
Here you can see the Inchworm Prerunner adapter mounted with the speed sensor in place. We reused the splined reluctor ring that trips the sensor, then added the Inchworm-supplied shaft spacer before slipping the female splined coupler onto the shaft. |