My 69 is a closed knuckle and I think your right that it was a transition year. Mine has the Rockwell T-221 transfer case.
What does yours have?
Seems like most of the 69 trucks had the Rockwell, but not the Blazers.
Looks like were all in agreement that no 67-72 factory 1 ton 4x4's. Here is a pic of my good friend's 70 1 ton that he converted to 4x4. He did a great job.
More of it can be seen at
http://www.j0ker.org/~yukon/tonys70.html