How can I replace my Dewalt DC759 keyless chuck?

I've taken out the left handed screw within the chuck and tried the allen key with a hammer technique but got no-where. The whole gear assembly spins and/or the clutch engages even with the clutch set on the highest setting and the drill set on the lowest speed setting. I've disassembled the drill, as well, and couldn't see a way to release the chuck. I'm getting frustrated. Please help!!!