I highly recommend you forego the master/slave all-wheel-speeds-the-same approach you are describing and do the following instead:
1) fix the mechanical issues first. see my
earlier post.
2) forget the PID and encoder for now. instead, program the wheel speeds properly using either the WPILib RobotDrive class or home-brew code described
here. This will give the driver full control of steering and strafing.
3) Once you've got that working, there's a good chance you won't need PID at all.