Look at the exploded view of the gear driven transfer case. Thin washer fits perfect and the only thin washer in any diagrams for the whole case. I think the only other washers are thick thrust washers. http://www.marlincrawler.com/sites/all/marlin/files/image-map-rf1a.gif
If you know better I am prepared to listen 
If you are referring to the component to the right of the oil pump screw with the black diamond beneath it, then this is the rubber o-ring that goes inside the oil pump screw (it is inserted into the inner diameter of the oil pump). The diamond indicates that it is a serviceable part (and we include it in our t/case rebuild kits
)
Taking pictures from our
4.70:1 Installer, there is no washer near the speedometer or oil pump screw...
...except for the flange washer that goes between the flange and flange nut ... but this is a thick, heavy washer that has no business being between a Crawl Box and a T/Case and does not look like any type of an internal case component.
You aren't referring to the inner snap ring that holds the bearings onto the front of both the input and counter shaft low range gears, are you?
(yes I know a snap ring is not a washer but there shouldn't be any other small component like this between a Crawl Box and T/Case)
Also would it happen to be a
Drain Plug Gasket that somehow / accidentally got left in there when you built the cases?
Here is a picture of a metal version of this gasket...
...which is possible as the T/Case (or any other drain plug that threads into an aluminum housing) uses an aluminum drain plug gasket as shown as the two gaskets in the bottom-right-hand-corner of our
t/case rebuild kit (one for the fill plug, one for the drain plug):
(hard to see because they sort-of match the white background)
Please let us know, and also most of us will be busy this Thanksgiving weekend and may be slow to reply.
At any rate at least you found it and got it removed before it caused any serious damage!!
Regards,
BigMike
