Stuck in 5th

So I was riding around at lunch today and the transmission started acting up. At one point, I tried to downshift and felt the shift lever bobbling under my foot. After that, it would not shift at all. I believe it is now stuck in fifth. Luckily I was only a couple miles from home and apparently the 450 has enough power to start from a dead stop if 5th gear, albeit not very well.

The clutch appears to be working fine as I can pull it out of gear to stop and start. I'm not familiar with transmissions at all and I guess this will be my learning experience. But before I started digging in too deep, I was hoping one of you may give my some guidance to help save me some time. Any ideas what it could be? Will I be cracking open the case to fix this? Hopefully I can get this fixed before the spring.

Thanks.

Mike