Need to spec up a drive shaft - Not sure how strong it needs to be?

Definitely get the steel billet yoke for the rear, and a matching front would be good insurance also. But that depends on your u-joint size. I footbrake at 3,540 to 3,700lbs + and since I switched several years ago to Spicer solid 7260 joints, a billet year yoke and a custom driveshaft from a local guy, no problems at all. I believe the driveshaft is 3" and may be seamless tubing.

I switched to billet because OEM style rear yokes will eventually spread on you and the u-joint will pound out. I found out the hard way.