Well with a 4" suspension lift, a 3" body lift AND trimmed fenders you should be able to easily run a 38.5". Depending on how far you've trimmed back the fenders you can probably get away with a set of 40's. If you *want* to run the 40's then I'd suggest running Bushwacker cut-out flares. Before installing the Bushwackers put the 40's on & *then* trim the wheelwells and fit the flares. You can easily get away with 38.5's but to run the 40's I'd use the flares because they really let you open up the wheelwells to what you need and they'll also extend the body to cover the tires which Joe-cop always likes to see.
Late Edit: I forgot to mention in the original reply that you need to be mindful of just how large your tires get. 36" is really about the max for the stock running gear. The bearings & axle shafts just aren't up to handling anything much larger. Now if you're primarily running around town and don't really wail on it then you'll probably get away with if for quite some time, BUT if you take it off-road with 38's or larger and stock axles you're just begging for a tow home out of the woods, a really big repair bill and a story that you'll tell for years to come. 38" and larger tires should really be run on Dana 60's. If you don't want to swap to the larger Danas then you need to spend some bux on the stock running gear. Larger hardened axle shafts and bearings are a must and a full-floater conversion would be an excellent idea. Truth be told you'll spend more upgrading the stock axles than you will just swapping to 3/4 or 1-ton running gear but everyone like to have choices. Regardless of what route you choose you WILL need upgraded axles to run anything over 36" off-road and expect it to survive more than a season.