mystery part on 2004 Suburban

It sounds like you are describing the EVAP canister and vent solenoid, it should have a vapor pipe running back to gas tank, a vent hose running to the vent solenoid, and a purge pipe running to the purge solenoid. If the EVAP system is not working you should be getting some EVAP related OBD codes.

Thank you KenF. I think you're right. I do have a check engine light that i haven't had scanned yet. I did notice after messing with this thing that it has a gas smell so it makes sense that it is related to the fuel system. Any idea where I can get the hose fittings? One of the plastic fittings broke. The hose is intact but the nipple on the fitting snapped off. I'm thinking it will probably be a dealer only type of part.

You might want to hit up a salvage yard or a pull-a-part, you could get the canister and all hose/pipe fittings on the cheap. Take the time to replace the charcoal filter as well, they get gas-soaked over time especially if you "top off" the gas too much.