Motorcycle Repair: 1991 FXRS Vibration, pivot shaft, wheels and tires

Question
My 1991 FXRS has developed a strange vibration, like a shuddering. It feels like the wheel is rubbing against something. But here is the interesting thing - it is tied to the RPM, not the road speed. In any gear, it begins at about 2000 rpm, peaks around 2300, and is gone by about 2700 rpm. It happens as I increase rpms through 2300 and when the rpm goes back down.

I have checked the torque on the stabilizer links, the rear fork pivot shaft nuts, the front engine mounting bolts, and the engine to transmission mounting bolts. It does not seem to be an internal engine or an ignition timing problem since the engine performs well and the shuddering only appears at that narrow range of operation. The rubber mounts are tight, the frame appears solid, the wheels and tires appear in good condition, and the engine/transmission/rear wheel are aligned
properly.

As a background, I bought the bike new and have serviced and ridden it regularly since 1991. The problem does not seem to be getting worse, and in fact would probably not be noticed by a new owner.

Answer
Your vibration can be linked to a few different things, since by your explanation you have checked the others.
Your bearings might be going out in the swingarm. To check it, remove the wheel and shocks and see if there is any side play. Your bearing might be going out in the steering head. To check it, pull the bike over ont eh kickstand so the front tire is off the ground. Have someone else pull on the tire in a straight line with the frame. There should be no play.And last, even though the tires look good, you might have a bad tire. Usually its the one in the front.
Good luck and happy riding
Mike