Shims are available from the page I posted earlier (with the measuring instructions). You may already have some shims on there, many spring kits come with shims and you may or may not need the shims supplied. For example on a single cardan driveshaft, you normally need no shims when installing lift springs only, but you do need them with a CV (double cardan) driveshaft. Also, you may not need the t-case spacers I think you mentioned. Those spacers would tip the t-case flange down at a steeper angle while your problem sounds like the pinion flange is too low (i.e. its at a lower angle than the t-case flange), so you may just need to pull those spacers. Measure the angle difference and figure out how far its off and fix it.

Steel shims (like I make) can be bolted or welded on and since they are steel, they are much less likely to break like the low quality cast alumunum shims you often find. Other options are to cut and re-weld the spring perches on the axle in the proper orientation, or you could add a longer spring shackle to gain angle at the pinion flange.