Anyone have any ideas as to why only sometimes I can smell some of my exhaust coming into the cab through my AC vents.

 

Might have a small up-pipe leak. Not terribly uncommon. It's usually the only time I'll smell exhaust in the cab.

 

If you have recently added a tuner or monitor to the truck that required you to run a wire through the membrane of the firewall, you may have to check that. I had that issue. I have also noticed that I smell trace amounts of exhaust in my truck when it is raining. I have no idea what that could be though!