Spicy!
The Spicer 18 came with two common main drive gear-tooth counts, either 26 or 29, depending on the age of the transfer case and what it originally came in. This gear is what the overdrive replaces when installed, so you must make sure the overdrive's gear count is the same as the main drive gear it is replacing. Fortunately, both are still available through Advance, so if you've got the wrong one, you can easily swap it over. There is also a gear available to work with the Tera Low gearsets.
If these bearings are munched, it's time to add a set to your order list for replacements. Ours were fine, so we made a needle bearing/washer sandwich. Use grease to hold eight needles in one end of the little drive gear. Then pop in the thrust washer and eight more needles. Now put the other two washers on either end and then drive the gear and pin back in to the planetary assembly. | 
We needed to replace the 26-tooth overdrive gear with one that had 29 teeth, so we swapped the bearings over to the new gear and reassembled the gear/planetary unit. |
We then moved on to the back half of the unit to inspect the synchro and to remove the bits of the broken shift fork. To start, remove the four retaining bolts that hold the back cover plate in place, then the cotter pin, large nut and spacer, and finally, the bearing. | 
Remove the two hex bolts and separate the rest of the housing. You can now remove the shift rail, detent spring and ball, and the solid driver from the housing. |
Our Saturn looked like one or both of the synchro rings had been installed improperly, which eventually caused the shift fork to snap like the bad guy's neck in a Bruce Lee movie. When rebuilding and installing the Saturn, you must be sure the three notches on the synchro rings line up with the three shift dogs on the synchro block assembly. If not properly installed, there will be a gap between the overdrive and the transfer case when you try to install the unit. | 
You can focus on the shift mechanism. Inspect the synchro ring's shift ring and drive member for excessive wear. Pay attention to the shift dogs and springs when removing the shift ring from the drive member because they will love to jump out and hide under something if allowed to. |
Order all the parts you need, plus the seal and gasket kit, wash the housing, and wait. We would do this as quickly as possible to avoid lots of time spent forgetting how you took the thing apart. | 
When the new parts come, reassemble your overdrive, install it, and blast on down the road. Don't forget to replace all of the gaskets and seals. |