IMO, you shouldn't even think about SAS until you have driven a double locked, low range truck.
Cheese is right...I can go more places in my truck than I am willing to go with sheet metal and class.
I have made the choice not to cut up a nice truck and put the same $ into building something out of tubing to go any farther down the trail. Why put $5 - $10k into an IFS truck and reduce its value? Put $10k into a buggy and have another vehicle worth most of what you put into it.