A electric winch will really load your electrical system. These tractors have puny alternators. There are some good batteries that would help you, like a yellow-top Optima. It was designed for hi-discharge.
If you wanted to spend some $$$, there's a couple things you could do. First, upgrade you alternator to a Premier Alternator/welding system. This would give you 200A capability, compared to the Kubota unit which has something like 10 amp output. This system gives you 110vac, a lot of current at 12vdc, and is also a arc welder. It is very popular with hard-core 4-wheelers, because it has enough output to run lights, winch, and you can weld the part you just broke while crawling over rocks.
For a winch, it's hard to beat a Warn 8274 for a electric version. Tough, tough winch!
The Milemarker hydraulic winch might be nice; it would seem to reason that if they can run that winch from a power steering pump, then our tractor hydraulics could easily run it.
Then, there's alway the hi-lift jack. It can winch, pull, push, clamp.
Look at these:
http://www.warn.com/Home/
http://www.premierpowerwelder.com/index2.html
http://www.milemarker.com/
http://www.optimabatteries.com/main.htm
http://www.rescue42.com/jackmate.html
RobertN in Shingle Springs Calif