So I would be getting better highway and less city?

If the highway is flat, yes. But on a hill you will find that you need 3rd gear more often than normal, which means worse mpg. But a lot of people here run 31's and have 3.42 gears. Its definitely doable.

Dont the tuners need YOUR pcm so they can look at your fuel trims and O2 crosscounts?

Some tuners give you basically a recorder that monitors how you drive and how your engine runs then they use that info to tune your PCM when you send both in but that's a lot of $$$. When you send in a PCM to a company they just give you a cookie cutter tune and match your Speedo to your tires.

What years axles would fit mine?
01 blazer 2wd