As a condition of re-enlistment can I ask to go to a unit deploying for Afghanistan?

You can ask but they will send you where you are needed. Be careful what you ask for. Combat is neither a game nor fun. It is a necessity and a reality of the military. While running to the sound of fire is what is expected when called upon, it is nothing to be trifled with. Your time will come when it does. The nations bidding you will do. Until then do as you are told and serve where you must. God Speed.

I can not tell your age. But you have been out since 2011 right? Your pic looks late 20's early 30's. If so, try to go Reaper, Ranger, or SEAL. The fight will be going on for a long time.
How are your knees?
My 2cts thinking back.
Break a leg Cpl.

Highly doubtful, not many units ata lol are heading that way. You can try and ask, but the needs of the Marine corps come first and always will.