The carry-ons may be a bigger issue to worry about. Does the Hilton actually let you check them?

I've usually been able to get form O'hare to Downtown in about 45 minutes. (I once made it from Hyde Park to the O'hare in an hour, with a bus from Hyde Park to the loop...) If it's just a quick exploration of Chicago, getting off at Clark & Lake would be a good idea - closest loop stop to O'hare. If time is really short, you may even want to take a quick look at Wicker Park.

CTA has been posting some drive times on the walk down to the L. If you are rushed and see that the drive time is short, then a cab may be worthwhile (though somewhere in the $30-$40 range.) However, I have seen hour+ drive times on Wednesday afternoons.

As long as some craziness doesn't happen, secuirty lines should be fairly short that time of day, so you may make it back in time even if cutting it close.