Backspacing???

i have a 94 xj with 6inches of lift. i got a great deal from a buddy on 4 bfg muds 32x11.50 like new. i see all these different backspacings and i need to get new rims and i need to know which backspacing to get? i plan on going with the ltb 34x10.50 next summer or fall so what ever backspacing i choose needs to work with both types of tires. i have already decided on a 15x8 rim just need some help with the backspacing?????

Theres an old pic of mine at 6" and 33x12.5x15 on 3.75 BS it tucked in well

3.75 sounds about right. eace: