mine is the 4 cyl lol hence the 22re in the sig the flowmaster makes it sound like a healthy v6...it sounds good and is a huge improvement over the stock muffler

 

Idk about the rims, as far as being 15 inches they will work but it depends on how wide they are, but I think I've read that they will work but someone else correct me if I'm wrong

 

31x10.50" tires if you don't want to re-gear, No lift is needed for those tires, unless you want a raised truck (if so, go with a small body lift)
Rear locker will aide in offroad. I can do things in 2wd that would usually take 4wd now that I have a rear locker, but if you don't want to spend 600+ on a selectable (with a on/off switch) locker, you will have to buy an autolocker. They take a little getting used too, but they are great!

I would take some time, and just search through the website. There is literally hours of reading to be done. After you do so, you will understand what the heck we are talkin about

 

If your wheels are actually the alloy wheels and not steel, they should be
15x7, which is plenty wide for 31x10.50 or 32x11.50. If they are the steel wheels I think they're 15x6....31x10.50 maybe would work. But 15x8 or 15x7wheels are better if you're going 31x10.50 or bigger. With the solid front axle on your truck you need to be a lot more aware of what you buy if you buy aftermarket wheels, since the sizes that seem to be most common have too much backspacing and they'll rub against steering components on your truck. You need around 3" or less backspacing.

 

dude, your 84 should come with 4:11 gears right? you can spin 33's with 4:11 or 4:10 gears without a problem. plus if im correct the 84 and 85 have big finders so you should be able to fit without a lift although a 2 or 3in suspention lift is nice. your not oin to get great gas milage anyway considering the gears. my 85 only got 16 with 4:10's. and that was with 235/75/r15's.