It is a valve that is supposed to prevent gas from escaping from the tank in the event of a rollover. My 76 cj5 had one and I rolled it twice and it worked, I think its some sort of a checkvalve. The thing the other guys are referring to is a charcol canister and its in the front left hand corner and is part of the emissions system.

84 CJ7 304 HEI 35 BFG,s rear dana 60 with detroit locker,Viar compressor, fibreglass body, 2 1/2 spring lift, TF999auto, Warn 8000 winch, Plus a cool little trailer on 35,s. JEEP stands for just empty every pocket