It's REALLY crucial that the drivers be aligned properly. The drive is basically a big 3 axle diesel truck with siderods.
Even with a driver gear being one tooth off, binding can occur. Short of removing the rods, the easiest way to check is spinning the motor by hand at the flywheel and see if the mechanism is binding.
Also, straight DC can be applied directly to the motor leads.
It may be a pain, but you may have to disconnect your DCC. It's easier to work on the loco drive without the tender attached.
Rusty