Grinding noise during deceleration.

[Hammer Transportation]

Hello all, I recently bought a 2015 Kenworth T680, 10 speed Eaton trans, with a Cummins Engine 400,000 miles on the clock. While driving today, it started rattling and grinding really bad when going into 10th gear. While the rattling/grinding is happening the shifter will not go into gear. The gears seem like they’re doing too fast. Clutch brake doesn’t help. When I finally get it in gear, it grinds sporadically or when I decelerate. It seems most prevalent in a certain throttle position (50%-70% throttle). And, the speedometer slightly fluctuates +/- 1mph on cruise. Any ideas on what’s happening?