Ditto. You can just swap the third member.

When I swapped my gears I used an '88 2wd axle and with a 4.625 third member. The '88 axle was roughly a couple inches wider than the '85 4wd axle that I had. My rear width is pretty much the same as the front now. The problem with using a 2wd axle is that there is no nipple for the breather hose. I solved this by filing the threads down on a 3/8NPT nipple and JBWelding the sucka in place of the rubber mushroom breather.

If you cut the spring hangers off and replace them with those from a 2wd pickup, you can then do a SOA swap too.