stock 1999 gmc jimmy 2d 4x4 non-zr2. ive heard from a couple sources the largest i can fit is 30x9.5x15. then i was just reading something about backspacing, im not familiar with that term, what does that mean? is that the measurement of how far your wheel sticks out from the hub or something? anyways, i may or may not change out my rims, but right now if im going to use stock rims, and want to get the largest tire that wont rub, i should stick to 30x9.5.15? and has anyone ran this set up and experienced rub, if so, how hard did you have to push it wheeling to get any rub? i dont plan on maxing out my trucks capabilities but i do like to go on forest roads and what not from time to time, want the most traction/clearance without rub that i can get, without lifting it is all. sorry for this question being asked so many times, appreciate any info you all can give me

thanks alot!
justin