its good to know what youve got. as far as diffs go, here's the breakdown.
from '73-'76 it was corp.12 rear/ dana44 front
in '77 they had the d44, and an optional corp 10 front
in 79 they got rid of the d44 and only had the corp10 in front
in '82 blazers really took a turn for the worse, they had corp. 10's front and rear.
for more info check this link
http://www.coloradok5.com/specs.shtml#1973
and for even more check the whole cororadok5.com web site.
do you know what year your truck is yet?