Since the neutral and the ground wires are bonded at the ground bar in
the panel is it supposed to be a voltage differential between ground
and neutral in the outlet? If I measure a voltage difference in the
outlet isn't it something wrong there? Or is there a maximum allowed
value?

Actually, if the circuit is in use, a slight voltage differential is a good
sign. (Like 1 or 2 volts.) Zero voltage differential might mean they're
tied together too close to you.