Jack up the rear, pull the hubs and drums. There are 4 retainer bolts under the axle flange. Remove them. You may have to use a slide hammer to pull the axle shaft. Put at least 2 bolts to hold the brake backing plate on. If you pull the shafts out 6 or 8 inches, you will have enough clearance to pull the third member. You can then remove the bolts and pull the third member. You will probably have to get a shaft made. If your 4 speed has a companion shaft, measure from the center of where the U joint goes to the center of the flange on the third member. This will be the measurement the person that makes the drive shaft needs. With a closed drive shaft, the drive shaft prevented the springs from winding up. This required a pivot in front of the axle at the spring. This will have to be welded tight. Some time ago there was a good photo posted on oldgmctrucks.com of how to do it. I think it was posted by WDOFTEXAS.