It is retained by a snap ring on the inside shaft. It should just pop
back in as long as it is in the correct
position. They usually take a good whack with a rubber mallet or dead
blow to seat. I'd be worried about
how it came apart.

I've found the problem and ordered most of the required parts, except
the culprit; the axle retaining ring (snap ring) that retains the axle.
Apparently the failure of this $0.50  ($8.00 from GM) part is not
uncommon that is why GM has discontinued it's supply and insists that
you buy a new "retrofit" axle with a larger retaining ring and groove
for about $500. Since the previous one lasted 6 to 10 years, I just want
the retaining ring but GM won hear of it. Local drive line shops have
the sam problem.  Has someone else come up with a solution? One person
has told me the old axle groove can be widened to accommodate the
retrofit retaining ring (which has a round cross section instead of the
traditional rectangular). Has anyone done this?

b.t.w. This is the same front end used in the K3500

Thx!