'73-'87. You could also use up to '91, but it would have to sourced from either a 3/4 ton Suburban or an the R30 series ('87 style) truck, which are usually either Crew cab or Chassis Cab.
Alternitavely, you could just swap the spindles, if you're only after the disk brakes. You'll have to swap out the balljoints (I think only the lowers on 3/4 ton, have to check the Moog book to be sure) and the outer tie rods. Other wise, it's a bolt on.
