If its not a DD or if you can wait until you get the SYE installed I would wait. If thats not possible you could always make an educated guess, if you know how much longer the 8.8 is and how much shorter the sye willl make things. I can tell you that a 2.5L with an auto, a D-44 and, and a dirty fabworks T-case skid, a 4" suspension lift, and a teraflex extreme short sye uses a 21" ds. With all of the changes that you are making I would just see if you could wait until you are finished with everything else.