I think it was mentioned on here, that towing by the bumper (without utilizing a bumper with frame tie-ins...apparently the rear bumper doesn't actually bolt to the frame???) is a bad idea altogether. It's extremely stressful on the bumper and can actually rip them off.
Sorry, kinda vague...I don't remember the discussion entirely.
Regardless, best advice is...if you have a trailer that has any real weight to it, you should just fork up the money for a proper hitch.