The pitman arm that came with your 6" lift is most likely the standard drop, which really isn't enough for 6" brackets and coils. Take a look here:
http://www.therangerstation.com/Magazine/winter2008/steering_tech.htm
About one third of the way down is a comparison picture with a stock, standard, and extreme drop pitman arm. Yours is probably the middle pic.
If you don't have it already (and I'm betting money you don't) you really should get the extreme drop (Skyjacker part number FA600) if you plan to run the 8" coils.
I have the FA600, 8" coils, 4.0L V6, supercab, and heavy winch bumper and I have enough bump steer to drive me nuts. My guess is it will be worse on your vehicle cuz its lighter.
Your BEST bet to address steering would be to get the "Stonecrusher" steering setup (about ~$100 more than the FA600 pitman arm) or modify Superlifts "Super-Runner" kit (about ~$200 more than the FA600). Search around on the forum and you quickly find out about both kits.
If you swap coils, that won't be the only thing you have to spend money on, just so you know. You'll need the FA600 (you can get by without it, but its gonna drive like hell) or equivalent equal length tie-rod steering kit( I.E. the Stonecrusher or Superlift kit), and an alignment (absolutely necessary).
Coils: $250
Alignment: $50-120
FA600: +$200