Remove the differential cover and inspect the ring gear carrier, if you see clutches,a locker or a spool it's positive. If the differential is still in the truck, you can raise the rear off the ground, spin one axle and if the other axle rotates in the opposite direction it's not positive. Hope this helps
Are they 1/2 ton? 3/4 ton? You may need to get a conversion u-joint if the axles are not the same. You might need to check how the parking brake cables are run since they changed sometime in the 80's. The '77 will have an intermediate cable that runs up one side of the frame to the cab, goes across and comes back down the other side with a cable at the cab that puts tension on it. On the newer style, the pass cable comes across the axle and both cables attach to a bracket on the driver's side. When I swapped my axle out, I put it on a floorjack to roll it under the truck. You will want at least one other person to hold the drum and guide the spring pads into position.
