I beleive 265/75/r15 is equivlent to a 31in tire

^Yes, normally that is the accepted thought. Normally a 245-75-16 or 265-70-16 is roughly a 30 inch and 265-75-16 is normally about a 31. Mine measured out to about 31.4 inch diameter.

Will I need to trim anything for 31in tires? (2 doors 89 S10 Sport 4x4)

i think it all has to do with hwo wore out the torsion bar is. i have 31s in mine with no rub at all when tund to the left but it its turnd all the way to the right than it rubs very little. thinking it might sit lower in the right side cause the bar might be wore out more but havent looked at it yet to see for sure

its either the tires they run or the rims they are running too, im not saying forsure, but some of the aluminum rims sit under the truck mostly, where you can get steel rims and they almost look like they are sticking out a tad bit... not saying forsure, but that would be my guess, width of tire too...