You can start here http://www.4x4wire.com/toyota/4Runner/tech/suspension/

For 3-6" you'll have to adapt a Procomp lift from a Tacoma of the same vintage to drop the front axle that much. The rear will require something custom with FJ80 springs, dropped panhard, different top arms at minimum and some custom length shocks. For shocks you can ref http://www.larsdennert.com/misc for specs on shock lengths for different brands. The XLS has many brands and the MX6 only PC.

Lars,
Gold Coast Cruisers |My 4Runner