only thing I can think of is maybe with out the lift there is interferance with the axle or maybe the driveshaft at full flex.
Anyways, if you want a complete system, I say go for the Trek's. I really like the looks of the Poly kit, but your still gonna have to do something about the gas tank skid. I'm still trying to decide between the two myself.
SOLD-08 Unlimited X, "jeep green", auto, limited slip, hushpower muffler, Aerotech Interceptor gauge,Powerstroke tranny cooler, skidrow evap skid, AFE Pro 7 filter, Teraflex leveling kit, EVO control arm skids, 15x8 Black Rock 909's, 33" KM2's