Wally,
normal cranking batteries are better for winching as can supply heaps more amps, far in excess of deep cycles or your alternator. having parallel batteries help as can supply more amps, as does 24v winch as they draw less current. (actually for the same load 24v draws half the current beacause current=volts/resistance.)
deep cycle batteries are really designed to be drained and charged on regular basis, not too sure if sitting for 3 months might screw them in the long time also.
have you considered some sort of rotation of batteries from the comp vehicle to daily driver to keep them happy. might be too much of a pita tho. might try asking someone at decent battery shop, they might be able to give better advice. or possibly car race teams, would they have similar problem with possible large times between car useage? lower CCA tho. another idea is i think there is a combination deep cycle/cranking battery, maybe optima
p.s. check manual or wotever for worst case current draw for winch, take this info to battery place so they know what battery for the load
btw, rear locker is bloody good. took whole family for a drive along some fire tracks last week, wife, 2 y.o daughter and 10 week old twins. no problem (thanks to diff)
cheers
Lance