Try 2W VHF no-license MURS Service (which has no ERP limit).

Two 2W VHF rigs will perform as good as 5W GMRS UHF rigs in your
situation, when comparing like-gain antennas, and the batteries will
last twice as long!

(Oh, and don't even bother with those cheap-ass FRS/GMRS
integral-antenna hybrids, those are junk... I'm assuming you were
talking about a REAL GMRS radio with decent receiver specs and
replaceable/upgradeable antenna).

For more information on MURS:
http://groups.yahoo.com/group/MURS-OPEN

- Stewart (N0MHS)