Do you have shocks for a 6" lift? Short arms greatly restrict your flex, so 4" shocks on 6" and short arms might work for a while, but with long arms, you are going to need new shocks. At that point, the only thing you dont need from a complete kit is the track bar.
If you dont need or dont want to use the correct shocks and are just wanting arms, RC sells just the long arm upgrade kit. I believe IRO has an upgrade kit as well that just includes what you need to go long arms on your existing lift.
2002 Overland - 4.7 HO, "Select-Drive", 4" RC long arms, TAG front winch bumper, 32" BFG KM's, 3" exhaust, Magnaflow hi-flow cat, Flowmaster 50 series