First off, I am going to get new tires at the end of the season. These are pretty much done. The rear is a little bit squared off from long commutes. I am thinking this is causing the handling issue. When I lean the bike over with some throttle, the rear drifts/slides a bit under me and then grabs again. It feels like a lowside is coming and then I have tire grip. When I downshift to stop, I lose traction and the back gets a little wiggly. Feathering the clutch tames the beast.

I'm just wondering if this is textbook symptoms of a squared off tire or if there's another issue at play. It's definitely tire related, but I want to know what is causing what I'm feeling.

How old is the rear tire?

I think you're riding my old M900 (black frame and wheels and red tank) and I used to ride those rears to the cords and never had what you're describing happen.

I have, however, had that happen with rears that were old and/or had all the oil cooked out of them

Are you blipping (rev matching) on your downshifts? The sliding on the turn however indicates that you are either out riding the tires capability or were crappy tires to begin with. Your rear shouldn't be sliding with a little bit of throttle since it stabilizes the bike as you roll on the throttle. Unless your rear shock is sprung at some ridiculous stiffness that your rear tire is trying to act like the suspension and sliding out.

ML - I am still on your old M900. These are the tires that were on it when I bought it. I THINK these tires are at LEAST 4 years old, and I have well over 4000 miles on them. They're toasty, but cords are not showing yet. I usually ride Michelin Pilot Road IIs and these are Metzlers. The suspension is set kind of soft and springy, so I'm guessing the tires are just tired. Even on downshifts, I try not to let the RPMs hit above 7k (the rev limiter engages at 9k).

I think i figured it out for you.

The only time I've felt issues like that was when riding with a rear shock that had blown its rebound, and was ready to be rebuilt.

For corners, the spring/compression does its work getting you set up, but without any rebound in the shock, there's nothing that prevents the spring from expanding back to its fullest as quick as it can. The result - a "squirrely" feeling into corners.

I vote for squared off tires. When it hits the bead that still has more tread it definitely feels wiggly. Happened to me.

The only time I have ever had that feeling, was when I was low on tire pressure.