P.S.

You might want to group the back wheels and casters so that in total they consist of four groups, so that they can move in a realistic way, eg:

rCasterWheel
lCasterWheel
rCaster
lCaster

The way the grouping is set up at the moment, the casters can't turn on the yAxis. It's up to you. If you are happy with the casters always pointing in the same direction, leave them as they are. Or you could make a slight compromise and do it as two groups "rWheel&Caster" and "lWheel&Caster". That way the wheels won't spin on their xAxis, but that would only be obvious in an animation with a tight close up on the wheels, and the casters could still be pointed in any direction.