For the front shaft, it might help clear the front crossmember like the previous poster said. For the rear, I'd put it on the diff end. The reason being is that the bearing supposting the pinion is much bigger than the bearing supporting the output shaft in the t-case.
If you put a thick spacer (1" or so) on the rear diff, wouldn't it increase the leverage on the pinion and possibly increase axle wrap??? Personally I like the Breeze ones.............two 1/2" spacers........one for each side of the rear shaft.