it's kinda of twofaced. a locker will ALWAYS give you more traction simply because of the fact that instead of having the tire with least resistance turning, it will cause both tires to turn. so if you have 1 on ice and 1 on dry ground, it will allow you to move on.
BUT if you have a locker and bad tires, then depending on the situation (most likely in offroad situations, or on snowy streets), the locker will give you the power to both wheels, but because the tires don't have anything to "BITE" with, they will just spin. so now instead of 1 wheel spinning and not going anywhere, you'll have 2 wheels spinning and not going anywhere.
now on the other hand
if you have very good tires with very good bite, even with an open differential you'll have more traction because the tires can actually hook up. then at that point, the locker will be a bonus making the tires essentially twice as effective because now you have 2 tires with VERY good traction pushing/pulling the weight of the vehicle.
so technically it works BOTH ways. the locker helps the tires, and the tires help the locker.
i was wheeling with a friend last winter who was in the process of building up his new jeep. he does have experience wheeling, but like us all has to build his jeep in stages because we haven't been doing so good getting the money tree to grow yet. LOL
anyways, we were on a winter run and he had brand new goodyear at/s tires. i had warned him about them, but he figured he'd use them till they were bald and then get better tires. we ended up strapping/winching him up almost EVERY incline because he couldn't get any decent bite in the powerdery snow. at the end of the day he ordered himself a brand new set of super swamper TSL (or one of those. don't remember exactly which ones). and the following weekend we went snow wheeling again. NOW...with the new tires he made 90% of the climbs on that trail where before he couldn't even get 1/2 way up them. the difference in traction JUST WITH TIRES was amazing. then several months later we also added a locker in the rear because he needed a new carrier anyways (blew up axle and build new one). now where before he had to use throttle and heavy wheel spin if he lost traction, he now almost idles up the same obstacles.
i would HIGHLY recommend starting with tires. it will be more of an "instant" improvement depending on what tires you have now. otherwise if you get a locker and your tires aren't up to the job, then you'll end up having to buy tires immidiately. so if you get tires first, that'll give you some time to save up for a locker.
