i would do the 4wd swap first.
Yes, theoretically a 4wd with open diffs vs. adding a rear locker gets you the same amount of "driving wheels" (2) there is a difference. In 4wd, you have a front wheel getting traction which helps when trying to get over a log or going up a hill or something. By adding a rear locker, just the back 2 will be getting traction which won't do you much good when your front tires are against an obstical like a log or rocks.
and btw, i did both and love it!
my $.02