I just did this and it almost sounds like a bad bearing for the hub pinion. I checked the gear oil and it seems to be fairly fresh and it was real close to full.
I guess next step will be to pull the rear wheel to get the hub out. And then do a spin check on both the driveshaft and the hub. If it is the hub, where might in be able to find bearings for it? I have a Suzuki shop manual so I will probably rebuild it (with likely a Eblag replacement installed while I do) but parts seem to be a challenge on these bikes.
I know oldbikebarn has some basic wear parts like pads, and clutch discs, but I also know they don't have these bearings listed.
Again thanks for all the help here, basic items like carbs, and plugs I'm more than confident with, but deeper dives in this bike are still new territory for me.