When I pulled my IRS unit apart I noted shims between the differential and the differential side brackets.  I failed however to note their thickness and specific location.  Today when I was fitting the tie bars to the wish bone pivot rods I found that there needs to be a total  .03" of shim, front and back, between the side bars and the diff housing in order to get the tie bars on.  I came up with the .03" # by bolting one side bracket up snugly to the differential and bolting the other side bracket on loosely.   I then installed the wishbone pins and  tie bars on both the front and back.  From there all it took was a feeler gauge between the bracket and the housing to determine the gap .03".  Armed with this number I split it  and put .015" of shim between the brackets and the housing on both sides.  I then mocked up the system and began taking measurements.  The drivers side hub was sitting 1/2" further back than the passengers side hub.  Due to all the measurement I had previously taken I knew that my mounting system was true and accurate and that my differential was sitting squarely in the middle of my sub frame.  The discrepancy must be in the Jag pieces.  I also knew that the side bracket  shims had been designed in to the system for a reason and figured evening up the hub location must have been the reason.  I removed the .015" shims and then put a .03" shim on the front bolt of the passenger's side, side  bracket and a .03" shim on the rear bolt of the driver's side, side bracket, leaving the other two bolts with out shims.  After I torqued it all down I measured it again and my hubs where with in 1/16" of each other.  much better than 1/2" difference.  I was worried that shimming them opposite like that would effect the way the tie bars and alignment plate bolted to the assembly, but when I bolted them up I found they fit better than they had when the .o15" shims where used on both sides evenly.  Seams kind of silly in retrospect but at the time I had an incredible sense of accomplishment as I had completed  the first step of dialing in my suspension.