Output shaft play 1980 XJ650

[Brandon12V]

Been getting a nasty vibration(from the rear) in 4 & 5th gear around 45-50mph. I figured it would be the U-joint so I jumped right in. For the record, final drive gear has new motul 75w90 fluid, the u-joint was found to have ZERO play, shocks are new, and no bolts or misc nonsense was found to be loose. everything looked pristine inside until I decided to check the output shaft play. This is my first XJ and I'm still learning, but this seems pretty excessive. 1980 XJ650 with roughly 17-18k miles....
Also the tires have 3 consecutive seasons as I ride year round. Tires were balanced less around 800 miles ago. It's not one of those "dead spot" vibrations. It's a harmonic vibration but more violent than the 60mph vibration normally present on these motors. Comes and goes away at speed enough to shake my headlight beam pattern enough that I can not only feel it in my seat but see it as well. Bike runs completely fine otherwise. Forks were rebuilt as well when I did the 750 front end swap.
the symptom is less noticeable under load through turns as well as down straight roads. It is not apparent with the clutch depress and coasting. It is most noticeable while cruising steady at 45-50mph. Getting on/off throttle feels a bit jerky, like a worn out motor/trans mount on 4 wheels. This happens in every gear.
Any insight would be appreciated. I've posted on XJbikes.com to no avail so far. Thanks

http://www.youtube.com/watch?v=pbNJ6nViZEk

[YOUTUBE]http://www.youtube.com/watch?v=pbNJ6nViZEk[/YOUTUBE]

 

[ApriliaBill]

Swing arm bushings???

 

[Brandon12V]

Haven't replaced them or the swingarm bearings but they looked & felt brand spankin new.

 

[kopcicle]

is that the stock header pipe ?

~kop ~

 

[Brandon12V]

The headers are stock. Collectors and everything else are not.

 

[kopcicle]

this may seem a stretch but check that the inner and outer pipes are attached not only at the head but at the collector . I had an inner head pipe fracture just ahead of the collector on a 1000 police and it shook the whole bike .

~kop

 

[Brandon12V]

Well it's all one piece aside from the reverse cones so nothing is moving in that department. I just used the headers and made everything from random bends and straight pipe stock. Still haven't figured out the issue tho. Put the tires on a bubble balance and they came out perfect.