First off if you want a mud tire why would you even want 18's? My first mod was to downsize to a 16" and wider rim. If your short on money get some cheap steel wheels (stronger for offroading anyway)
After the lift (2.5" OME spring lift) I wouldn't go bigger than 32's unless you want to do quite a bit of trimming/banging with sledge. And yes the stock shocks will work. Look into the TrXus M/T tire, they are freakin sweet! I think I have 45,000-50,000 miles on mine and they still have plenty of tread for the mud. I think I will get nearly 70,000 miles on them which is amazing compared to any other mud tire I have had/seen.
2001 Ironman Daily Driver... 3.5" suspension lift (OME912 springs and 1" spring spacer), ball joint flip, 1.5" front diff. drop, 33"x12.5 TrXus MT, 16x10 Eagle Alloy rims, Interceptor, PV muffler, K&N air filter, Alpine Supercharger, Bilstein shocks, and some trimming.
2000 Ironman Project LS-1 VX... very slow progress but someday....... ohhhhh someday......