I could be wrong about 99 models but i know that on the newest z-71 models, stock shocks are a larger diameter shaft and resevior then a truck without the z-71 package. So, just replacing them with a new style, stock, aftermarket shock, will give you more shock then anybody else gets out there when they replace theirs. Improvments, if thats what you want, may be getting an all gas/ or partial gas/oil shock combination.
tires: just opens up a can of worms. please note recent thread on this page titled 'best all season radials'. Personally i stick buy BFG'S All terrain/KO's or Michilins, but others will have there favorites as well. Jack