It takes a 17mm six point socket.
Right side tightened, compressing the rubber spacer, limiting crossmember movement.
The Advance Auto Parts "HELP" steering coupling disc kit, #31000 can be used to shim the stock, loose fitting crossmember mounting plate.
There is a lot of slop in the rear end crossmember mounts.
These are the stock "stopper diff mtg" as listed in Subaruparts.com.
This is the right side with the rubber spacer, before its compressed.
This is the right side without the stopper.