The thin washer goes between the two rows of rollers to provide the required end guidance. A hard washer against the gear case and against the layshaft provide the other faces. What is happening is the ends of the individual rollers are rubbing past each other. While not actually tripping or catching, it is causing the rollers to cant slightly, creating negative axial clearance, and possibly tight radial clearance (though I doubt they are able to twist that much). This is likely the source of the excessive friction you are feeling. Its getting jammed, with perhaps the rollers skidding around and not rolling as they are pinched. Unless the sleeve gear is like new, there should be a wear witness mark between the roller tracks hinting at where the washer(s) originally should be.
Do not operate the gearbox with the rollers abutting each other side to side, hoping that it will ease itself in. It won't. It needs to come apart and the rows of crowded rollers properly segregated. Narrow rollers need to be guided at both ends by running in a groove, between two continuous faces, or guided by a cage.
Of course, it is possible the excessive friction is still due to something else. But you really need to get that thin washer put in the proper place. The only reason I can think of that it would manifest itself after you placed it in the frame is simply time, or being pulled around by the rear chain is placing a side load on the sleeve gear and aggravating the issue with the rollers (or whatever the root cause of the friction is.)
-Doug
[Add more comments and clarify. 09Apr13, Doug]
