I bought a 2-1/2" Heartthrob dual system for my 2WD and am pleased with it. They also make a system for a K20s. The pipes are mandrel bent. Systems are available for 4X4s with behind the wheels or rear exits and with "stock" (#4149SK and or #1014149...actually fairly loud) or glass pack mufflers (#4149GP or #1024149). The part numbers starting with 101 and 102 are rear exit.
I got mine from Auto Accessories Garage with free shipping (
https://www.autoaccessoriesgarage.co...SABEgI42vD_BwE)
The Heartthrob website (
http://www.heartthrobexhaust.com/manifolddualkits.htm) says the system won't work with a Rockwell transfer case.
Perhaps someone here has some personal experience with the 4X4 system.