Ford super duty 3.31 rear end. Will it be a big mistake. ?

I run 275/R65-18 highway tires, so the 3.31 works (mostly) for me. My 800 torque and 400 hp works great as others have said above. Tow haul mode absolutely eliminates gear hunting—my transmission just drops to the gear needed and holds it. My comment earlier about dropping to fifth on hills was based on experience in states with rolling hills where little grading is done. My favorite complaint is I-26 in NW SC. The trans will drop to fifth going up, back to 6th as I crest, and shift back down to fifth going down with cruise control trying to hold the speed. Just a ton of shifting! Yes, I know the owner’s manual says to not use cruise when towing. I end up shutting cruise off going downhill letting it coast up a few mph.

Just to answer the OP’s question clearly: no you’ll not make a mistake with 3.31.