The 1pc d-line sits at a different angle than the original, so you have to rotate the pinion down to match the angle of the tranny flange. The shim goes between the spring and the perch, and run abot $12(when I purchased mine). Contact 4x4 labs here in GV, they may have the shims, if not Foothill 4x4 in Auburn should, I don't think Trucksmart will have them. Measure the amount you need, I used a 3* on my '90 x-cab, it has a 3" lift, so a stock rig may need less of a shim.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed