It's more of a late vs early model thing rather than 2wd vs 4wd. From around '79 on most (if not all) trans shells were cast with the bosses for the HD flywheel cover. 2wd stuff was still equipped with the tin cover inspite of the fact they had the undrilled bosses for the thicker HD aluminum covers. All of the 4wd applications came with the HD cover.
Take some time to familiarize yourself with the actual shells rather than applications. Go to the Webshots link in my sig and look in the parts album for example pics of an undrilled 2wd case.