Hey Curt...welcome back!
The vibration. Is it a high frequency buzz or a very pronounced shake? Can you differentiate the vibration from any suspension issues over possible engine issues?
Engine vibrations can be a bit hard. Sometimes it happens only under a load, sometimes it's just the engine. Try reving the motor in neutral with your hand on it. Try moving your hand to different places on the motor to see if it's more pronounced in one area over the other. This might help isolating it. if it vibrated in neutral like you are saying...most likely a crank, rod, or the bearings. If this is the case, I would not riding it till you get it replaced as when these go, the whole motor can be damaged.
If there is no vibration doing this, it could be a bad bearing or shaft in the transmission. OR...it could still be the crank/rod/bearings in the engine that only show up under load. I know..."then how do you know???" The remedy is almost the same. You're going to have to crack the case anyway so...
Next....suspension. Many times miss adjustments or loose hardware will show up as a vibration at certain speeds.
How is your alignment? If your alignment is off, it can show up like this. The whole bike will shake, most likely in the bars is where you will feel it the most. I know some people like to do crazy things with their camber/caster/etc... I have always set my bikes/SXS/car up zero on everything. They always work great...no issues. You have any...just let go and the bike/SXS/car straightens out on it's own. How about the chain tension? A chain can do this if it's not adjusted properly. Sounds elementary but...put it on a stand, taking all the weight off the wheels and suspension and shake each wheel up, down, side to side. Could be a loose bolt, bad wheel bearing, bad ball joint. Check the tie rods too for no play. Check the steering stem nut, excessive turning play and up and down play. Check the rear linkage to make surety hardware is tight. Basically...shake everything in every direction it could possibly move to see if something is loose or wore out.
I know some of this sounds like ...."well no shit..." but...sometimes we overlook things. These are all the things I would do before cracking a case. Once you crack the case...you better know how or have a deep pocket. Every time I have heard someone paying to change a crank or transmission stuff on a bike, it's always over $2000.
Feel free to hit me up in DMs if you want. I've built and worked on a lot of these bikes.