Excessive play in drive train

[Ercouper]

I recently purchased a pair of Rocket III's, an '05 with 55K mi, and an '07 with 17K mi..
Let me say that this is my first experience with Rockets so I am not overly familiar with their quirks, common problems, etc..
After riding both for a while, I find that the '05 has more "play" in the drive train than the '07, which I would expect due to miles driven.
If cruising along at say 40mph at a constant speed and increase and then decrease throttle setting, there is a very noticeable clunk, clunk which is very annoying.
The '07 does the same but is not near as noticeable. My question is.......is there a common cause of this? Is the U-joint suspect? Play in the rear end pinion and mating gear? Bad shock absorbing rubber inserts in the rear hub? Or.....??? Pardon the lack of proper terminology. Any advice on where to look first? Thanks for any help or suggestions.

 

[TURBO200R4]

these bikes need to be greased on a regular basis on the final drive (differential) splines. they like to wear out i do mine at every tire change
6 to 8000 miles. they can be replaced with later final drive guessing 09 up that have a lager spline drive shaft and larger final drive spline.
also be careful of the final drive drain bolt it breaks very easy.
you can do a search this site has a lot of info.