Regarding the shims going on the outside, between carrier bearing and the case..Drew is very correct. First, the outside shims are included in the ARB box for that reason..the ones that would be used betwen bearing and the carrier do not fit anyway (bearing ID larger on the ARB dif than on Dana 44 carrier) AND, if you did place shims between the bearing and the carrier, it wouldn't take many to cause the tiny air supply hole to be misaligned with the slip ring groove where the air passes through. This will leave the ARB with a leak and possibly make it inoperative altogether.
As for the spreader...(I set up a lot of ARBS)...I make two wedges cut from oak and drive them, one on either side, between the carrier main housing surface (opposite the ring gear) and the case. That is a satisfactory way to spread the case and load up the bearing and shims on the opposite side from the wedges and allow the last of the shims to be installed between the other carrier bearing outer race (the side with the slip ring and air tube) and the case itself.
I should take pics the next time I set one up...I mean to every time, but,....%) In any event, every ARB dif I've ever set up is still going strong...some pushing almost 10 years of routine trail abuse.