with mods below, 3:73's, 305/70/16 good year MT/R's, 8000 lb. stock class. the truck will NOT run 3rd gear. tried it a few times, not enough power. i do not have a tc lock up switch, but i can say that when pulling, sometimes the tc will lock up and sometimes it wont, when it does it makes a big difference in ground speed.
also, i noitced that doing "boosted launches" seemed to help me get enough ground speed, quick enough to get the t/c to lock. if i just roll out easy and squeeze the pedal to the floor it never gets going fast enough to get it to lock. my truck dynoed in august at 328hp. maybe with 400 you could run 3rd, but not 328.
hope this helps...