Going to be ordering your Extreme duty Driveshafts for a '66 Bronco, That has the Borg Warner U-Joints. I will need to order 2 of these, correct? 1 for front & 1 for rear?

Yes, if neither drive shaft has been upgraded, you will want to replace both yokes, and order 2. Sometimes the rear has already been changed, so it is best to measure both first, unless you want to play it safe or know for sure.