Gas tank does NOT need to come down. Un bolt the 4 bolts on the diff. that hold the driveshaft on. Then un bolt the diff. from the fram of the car and pull it out of the way. VIOLA! Slide the drive shaft out. Thats how I got mine out and at first sight, I also thought the gas tank had to come down.
Be sure to use lok-tite, or equivilent on the 4 bolts that hold the driveshaft to the diff. Mine weren't even finger tight when I pulled my driveshaft out because there was no lok-tite on them.