what size rims are you wanting to run? i can tuck most rims on my truck. ive run 18x8 25mm offset Nissan titan rims, 17x7.5 30mm offset (yeah 5" BS) Toyota Tacoma 6 luggers, Nissan Supercharged Frontier 17x8s and various aftermarket rims, like Center line 18x8s with 5" backspacing. its Close on the fronts but the rears are waaaaaaaaay inward. Isuzu rears have a 1" overall narrower track then the fronts, so you have an extra 1/2" +/- on each side. Also 18x7 Isuzu Vehicross rims tuck ALOT!!!!!!
Shorting a rear thats not a Dana/GM corporate or any other "muscle car" rear is going to be $$$$$$$$$$ you can have the tube cut and shortened and have the ends rewelded on and probably have the axles cut down maybe an inch, if there is enough spline length......but to gain an inch on each side........thats about all i would think you could get out without having the axle shaft splines reground/regrooved.....