its the internet everyones a professional on here

they work together to center your axle and adjust your caster. Uppers will do you just fine adjusting your caster.

 

Damn, now that just makes total sense when you think about it. I suppose the lowers are slightly easier to replace which is why I've seen most people on the forum recommend changing those.

 

You have to cut a bolt on the pass side stock upper (or remove the muffler to get it out) before you can install the new uppers. Not a big deal. People get lowers just because they have a better range of adjustability, and aren't all that much more expensive.

And as FT mentioned, getting both uppers and lowers gives you the ability to move the axle forward/backward in the wheel well. (becomes more important the higher you go...)