Is the bearing not in the case? In general this is how you go about it.
1 - Case bearings installed and case installed.
2 - Clutch side retainer bolts out of the tunnel, clutch side bearing presses on.
3 - Bearing spacer and o-ring on the chaincase side of the shaft.
4 - Put the track up in the tunnel, put the drive shaft up and through the clutch side hole and shove it through as far as you can so that you can rotate the case side up.
5 - Slip the shaft through the bearing in the chaincase while pushing the track up into the tunnel enough to get the drive cogs to skip the teeth so you can slide the shaft in.
At this point I put the lower gear and bolt on to hold it in place while I bump, shove, swear and cuss getting the shaft all the way in to the chaincase so it bottoms against the bushing. Once it is fully seated I tighten the lower gear bolt down to hold it in place and then put the clutch side retaining plate in the tunnel and bolt it down.
sled_guy