Well the posts seem to all be going in the direction of replacing the rear shock, and I agree. I hope you find the funds to do it soon. I have 2 personal experiences with behavior similar to what was described in the original post. First was caused by Givi cases mounted too far back on the bike, and the absurdly soft and underdamped stock suspension on the Suzuki Bandit 600. Second was on a racetrack on a YZF600R at 140mph. This was repeatable lap after lap in the same place on the track. 2 clicks harder on the rear rebound damping adjustment was the difference between some wiggle and no wiggle. These may be different circumstances and different bikes, but I think the causes of wobble are the same with any motorcycle - baring any mechanical problem, it seems to go back to the rear shock being the primary culprit, with a spring that is too soft and/or rebound damping that is too weak. I guess the theory is that with the rear end bouncing around the castor angle is always changing, and that castor angle is one of the fundamental things that makes a bike balance itself.

What's interesting about bikes is even though they can be so unstable with the wrong setup, they are amazingly stable with a decent setup. I can't count the number of times I've experience some road hazard that destabilized the bike and then felt the bike recover from a big wobble in less than a second or two, even at really high speeds like those on a race track - as long as one keeps their arms relaxed and lets the bike's geometry do its thing. Well that's my slightly philosophical thought for the day.

Good luck fixing your problem.