I don't think your exhaust is hitting the vent. Look for rub marks on the control arms ot other areas. Sometimes the exhaust can work itself loose and you have to get under the car and re adjust everything.
+1 for flstang65, my exhaust was hitting my rear control arm, i have a bolt up slp cat back so there are lots of places to adjust and get the right fit, if you have a ball and socket system it may have just worked loose or a hanger my be busted or something... but i doubt you are hitting the axle, but if you are it's probably still because of exhaust location, so try to re-fit the pipes before you cut anything up
i have the same problem with my exhaust, and i'm pretty sure it's hitting that vent. the top of the vent looks like someone's hit on it with a hammer, and there's some marks to match right above it on the exhaust pipe. i've checked before and i don't see any signs that i'm hitting a control arm.
i've considered putting a vent in my diff cover and removing that one, but haven't gotten around to it yet. it's annoying as hell, whenever i hit a bump i hear a loud metallic bang from the back end.