I have a 98 tracker 2 door, Ive ordered the 2" OLD MAN EMU with 2" body spacer lift from LOW RANGE OFF ROAD . My question is what back spacing wheel measurement will fit a 31"/10.50/15 Goodrich mud terrain km2 when suspension is stuffed in fender well? I want as wide a stance as possible yet will stuff when compressed. I'm hoping only inner fender trimming will be needed but maybe you guys can enlighten me.  I believe a 3.75 bs is ome and I've seen guys like a 2" back space for the wider look but think that the 2" might get the tire into the fender some?
Thanks

Tough call.  The more backspacing you put on the more likely you are going to rub.  3.5" of backspacing is usually a good number to go for.

 I run 1" spacer on some aftermarket wheels on my Tracker with a 2" lift in front and it almost rubs with 235/75 R15 tires, a good 2.5" smaller then your 31" tires. 

probably doesnt help you much but i run 31's on stock rims with 2-3" of suspension lift. i get rub on stuff on the rear end but they tuck nicely up into the fender wells, just needs some message and some extended bump stops to make it work well.
with the stock wheels the 31's wont stick out past the body (check your state laws to see if you can get away with tires sticking out or not)