I did add-a-leaf on my truck, the old front shocks went back in all right, had to compress a couple inches; somewhere between 2 and 3 inches, and the front doesn't sag much when lifted, so they should be fine.
the rears however will not go back in, at full extension they are still 1/2" too short. I even stood on the tail end and had my brother up there as well(say 400lbs on the end of the frame) it barely sagged, so if anything the bed will barely affect the level it sits right now.
can't remember the exact measurements, but the old shocks I think, are 14" compressed, and 23.5" extended. I think a 16 compress and 26ish extended would probably work. I'll need to measure sag, then find a way to measure compression, as I can't get the truck to really compress any more then a few inches, I twist the frame and start to lift the front....but as I write this, compression doesn't really matter, I can do bump stops to limit upward travel.
Is there a way to find a Monroe equivalent shock for lifted trucks, or am I stuck with going aftermarket, like skyjacker/rancho/bilstein(I wish)??