meehh.. I had a 35 on the stock carrier (sans third brake light) my tailgate never sagged. Let me brake this down for you the way I see it..
what is the actual weight difference between a stock 30" tire (or 235/75r15 tire) and a 33" tire... no that much, maybe 10-15 pounds at best. add a steel wheel instead of a aluminum one, another 10 lbs, so at most, 25 lbs more... if the tail gate is latched, 25 extra pounds is not going to sag a hinge, heck, 100 extra pounds on a latched tail gate is not going to sag a hinge, keeping it open for extended periods of time, might, but not closed. put it on there, you'll be fine