if it's the bearings, should be right at the knuckle.
bearings 20 - 60 bucks probably
http://www.bikebandit.com/houseofmotorcycles/2005-honda-trx400ex-sportrax-400ex/o/m2662
all i was trying to say is if the rear sprocket is moving or not, it's either ther rear end or at the crank... you should be able to somewhat feel it on the bike if it's at one end or the other... yes? no?