Are you sure you're asking the right question?
You've got a 2WD rig with over 200,000 miles on it. You should really be asking if you are truly dead set on spending the money on a 4wd conversion.
Jump ahead in time. Imagine yourself already with a rear locker and a winch. All things being equal, including driver skills, you'll never be able to match up in capability with 4wd rigs. I have first hand experience having wheeled with a 3rd gen 2wd rig running a doubler, ARB locker, and 33s. I was in a near stock 1994 4Runner with 31s and he was getting stuck. It's just not possible to equal the capabilities between 2wd and 4wd. If you want to verify that for yourself, get some of your 4wd buddies with a locker to do the 2lo mod and wheel with them. See for yourself.
A 2wd to 4wd conversion is going to set you back around 5k. Around $1500 - $2000 if you do all the work yourself.
If I'm gauging right, you'd be looking at someone else doing the conversion for you right? If that's the case are you sure that you'd rather put another $5,000 into a rig that will still get spanked by a non-locked 4wd? You would get much better bang for your buck to buy a 4wd rig with lower miles and spend a week swapping your mods. That's coincidentally about the same amount of time that you'd be without your rig waiting for the conversion.
If you're able to take a step back from the emotional attachment to the mods you've already done then you should save up and buy a 4wd rig. Save up sooner rather than later so that you won't take as much loss on the stuff you are not able to move over (like a locker).
BTW I have to agree with Sn0wrunner. Lockers are way over rated. They are not useless just over hyped. The number one excuse I hear for wheelers lacking driving skills is "If I had a locker..."
My rig is locked front and rear and I very rarely need them. Nearly every time I'm stuck, feeling so lazy that I don't want to back up to take a new line, and try the locker I'm still stuck. Anecdotally about 1% of the time, maybe not even that much, it allows me to go where opens wouldn't. Now that I'm thinking about it...sadly, out of the dozens and dozens of trails in the past several years, I can only think of one time where the lockers got me out of a jam that open/open couldn't. I slipped into a rut, was flexed out, and in deep snow.
But if you don't give a rats butt about my ¢.02 then go for the winch on the rear bumper. If you don't want to spend the cash to mod up the rear bumper then get a locker. A winch on the front would be my last pick. Winches on the front are fun (and look ooooh so cool) but they really are set up to help everyone else in your group more than yourself.