If you have the 5.4 engine, you have the 9.75" rear. The 4.6 and 4.2 are mated to the 8.8" rear.
And why do people keep getting so confused about the 'additive'? If you have a LS rear, just buy the gear lube labeled for a limited slip rear, it ALREADY HAS THE ADDITIVE IN IT! No need to 'add' anything! Just read the flippin' label, it's real easy. I've owned and been inside more than one LS rear axle, and I've never yet needed to buy or use any additive. If it's an LS rear, I buy the gear lube specified for LS. If it's an open rear, I buy the regular gear lube. Done deal.