What I've seen done with S-series trucks is to swap in a set of Jeep axles out of either an XJ, YJ or TJ. The bolt pattern doesn't match your current rear, so you will have to swap both. THere is NO solid front axle that matches your current rear bolt pattern.
If you ever plan on going bigger than 33's or are abusive on equipment, I'd suggest going bigger than Jeep axles and go with a set of axles out of a 1 ton Ford for the driver's side drop or you may be able to get a different transfer case out of a bigger Chevy truck to move it to passenger side drop. You will then mist likely have to re-do your exhaust because it will most likely be in the way. If you are looking at a SAS, you will be doing a LOT of fabricating anyway, so a new exhaust shouldn't be a real concern.