Nate.
Same thing happened to me yesterday! Hard brake from approx 50 to 60 MPH. Sliding to stop. (fluff trail base not ice) Upon acceleration. I could hear what sounded like a gear grind from chaincase. I had my riding partner lift the rear end while I lightly hit the throttle still had the noise. I checked the track for ice, all appeared to be ok underneath. I checked the chain tension. It appearred to be loose? I readjusted the chain and went through some forward - reverse shifts and the noise was gone and all appeared normal.
Oil level is ok, sled now has approx 1200 miles, chaincase was serviced at 300 miles. Drove the sled back to the truck 100 miles away and no issues after that. I will now drain that chaincase oil. I want to check for metal shavings. Sure wish the Yamaha dip stick was magnetized.
Dan.