I guarntee you dont have 5.38's if you did you wouldn't be asking this question. when I had 32X11.50' (same size as yours) on my rodeo it drove fine with the stock gearing.
sounds like your truck is screwed up. With 35's and 4.10's I never dropped below 60 it may have been in second gear at 5000rpm but it would at least go 60. <img src="/forums/images/graemlins/lol.gif" alt="" />

94 Rodeo 3+3 lift Red)
95.5 Rodeo 1" Body lift Dana 44' front & rear 4.88 gears 35"s 176,000 miles Optima Yellow top InexAir Co2
front winch bumper custom rear bumper, Hella 500's
2004 Silverado duramax