Thanks for the visual assist, Tim - saves me hunting down my instruction sheet (which doesn't have any pictures anyway).
AngryDog: Assuming that your XJ is completely stock, if you look underneath right now you'll only see the crossmember, and it'll be bolted directly to the frame rails on either side of the vehicle.
The spacers in TIM_MN's picture are the lowering kit - it's for the transfer case, and isn't required when installing that skid. You should only have that if (1) you have lifted your Jeep and have encountered driveline vibes, or (2) you have a bone-stock '97-'01 model with vibes, in which case someone may have installed one as a fix for the problem.
Any more questions, let us know.
Rob