All Sierra rear calipers are the same, (GLS, XR4x4, 2wd Cossie) EXCEPT for 4x4 Cossie - as mentioned the 4x4 Cos caliper body is wider to accommodate the vented disc, so is not a direct fit as a replacement on a 2wd car with solid disc.
You can however fit all the 4x4 cossie parts together on a 2wd car, ie. calipers and vented discs.