transmission lockup Vulcan 1500

Had a disturbing thing happen last week. I was riding my '97 Vulcan 1500 Classic with wife on back, and was traveling approx 30mph, when I heard a racheting sound(akin to missed shift rattle), 2 loud clunks, and the the rear wheel locked up ! There I was, in the middle of the road, unable to budge the bike. The shift lever will move, but does not change anything. I have not yet looked at it, it's been raining & I have no garage. Has anyone out there heard of this happening? or have any suggestions/info they could share? This could have been alot uglier scene had I been going 65mph, as I had been just minutes before this happened.