Hope it helps
Trail-gears info helped me understand it:
Link:
http://www.trail-gear.com/assembled-t-cases
1979 -1995 TOYOTA PICKUP AND 4RUNNER APPLICATION INFORMATION
From 1979 to 1995 Toyota used at least 12 different individual transfer case in Pickups and 4Runners. In choosing a Trail-Creeper replacement transfer case it is important to understand the differences between the cases and how to choose the correct case for your application.
Gear drive vs Chain drive:
Transfer cases that use a chain to connect the front drive are called "chain drive" style transfer cases. Transfer cases that do not have a chain and use direct gear contact are called "gear drive" transfer cases. Transfer cases installed into trucks with automatic transmission, and/or V6 engines are usually chain drive style. Some later model 4Runners with a manual transmission and a w58 transmission also have a chain drive case. Determining if a transfer case is chain or gear driven within this year range is easy to do by looking at the rear cover of the case and counting the number of bolts holding the cover in place. If the cover has 7 bolts (as shown in photo) it is a gear driven case. If the rear cover has 5 bolts then it is a chain drive style case. At this time Trail-Gear does not support any of the chain drive applications.
Top shift vs Forward shift:
Gear driven transfer cases that have their shifter come up out of the top of the transfer case are known as "top shift" style transfer cases. Some transfer cases have a block off plate installed on the top hole and are shifted via shift trails and a shift lever located at the rear of the transmission. This style of transfer case is known as a "forward shift" transfer case.
The rear case of a dual transfer case assembly must always be a top shift style case, otherwise it would not be possible to shift the rear reduction housing or 2wd/4wd sections. The front reduction section of a dual transfer case can be either a top shift or forward shift style. Normally a forward shift case is used with a forward shift transmission. Toyota uses two different reach shift forks, 4.5" and 4.0". It is important to have the correct shift forks for the transmission used or the transfer case shifter will not work correctly.
21 or 23 spline:
All 1986-1987 turbo applications use 23 spline gears between the transmission and the transfer case. All other gear driven applications in this year range use 21 spline gears. The difference between these gears is that the 23 spline parts are a little larger and thus stronger than the 21 spline parts
If you are installing a dual case into a vehicle that has a 23 spline output transmission (such as a turbo or V6) it is recommended that you use a 23 spline dual case adapter and a 23 spline rear case input gear.
On applications that use a transmission with a 21 spline output you must use a 21 spline input gear on the front reduction housing of a dual case setup. On this application it is possible to use 21 or 23 spline gears between the front and rear reduction sections. If a 21 spline input gear is used on the rear reduction unit than a 21 spline coupler mused be used on the dual adapter. Likewise if a 23 spline input gear is used on the rear reduction unit than a 23 spline coupler mused be used on the dual adapter.
We strongly recommend that 23 spline components be used between the front and rear reduction units on all dual transfer case assemblies. The 23 spline hardware is stronger than the 21 spline. This is not a problem at the interface between the transmission and the transfer case with a stock or mildly built 4cyl motor. As engine power is delivered through the first transfer case into the second case the torque is multiplied by a factor of 2.28 when the transfer case is in low range. When both reduction sections of a dual transfer case are engaged there is 2.28 times as much more torque placed on the rear input gear/coupler as compared to the front input gear/coupler.
Your runner is coming along nicely! 
:beer: