I'm planning to drop the rear of my 99 2wd supercab to level it out. Looks like the best way to go is lowering shackles. Do I need to change to a shock designed for lowered vehicles or will the stockers be ok? Also planning on getting some 20" rims, what suggestions do you guys have for tire size?

 

Yup... Shackles are the way to go. As far as shocks, I've heard the the stock shock is fine with a 2" drop. It's up to you whether a performance drop shock would be worth the added expense.

I'm running 275/45/20 on my 3"/3" drop. With only a 2" rear drop, you might some more sidewall height. Try using this tire calculater to determine what 20" tire might fit your needs, while closely matching your stock tire size.

http://www.miatanet.com/garage/tirecalc.html

 

I agree, you should be fine with the OEM shocks. With only lowering shackles, I'd go with something like a 295/50-20 or a 305/50-20. Personally, I'd go with 22's. If you don't you will probably wind up wishing you did.