The main difference with the 99-04 and 05-06's is the change from leaf springs to coil springs. The coil spring trucks have the hubs,(where the wheel bolts on) spaced out farther to allow space for the coils. This changes the offset on the wheel to keep it inside the wheel well. If you were to put 99-04 wheels on a coil spring truck, they'd stick out of the wheel wells about 4". Likewise, if you put 05-06 wheels on a 99-04 truck, they'd stick 4" too far into the wheel wells. Hope this helps.