[Kevan]

Ideally there would be some backlash in the gears, you want to be able to rock the wheels back and forth a fraction without the crown gear moving.

As the wheels are up against the bearings the crown may be pulled tight against the pinion, pulling the wheels out slightly to give some axle side-play will help...another option is shoving the crown gear away from the pinion, the easiest way to do this is with two pairs of long nose pliers, one to hold the axle tight whilst using the other to gently lever the crown gear over a few thou'