Speedo sending unit attaches to the tailhousing of the transfer case.
There's several steps to removing the rear extension housing, I'll try to describe the pics as I can't scan in the FSM:
1) Remove detent spring and ball. Remove detent plug and remove detent spring and ball with a pencil magnet. (looks like it's on pass. side of rear case extension, about half way up the case).
2) Remove shift arm set bolt and remove bolt and lockplate. (looking down into shift tower)
3) Remove shift lever shaft plug, then pull shaft out with large magnet. (above output shaft on backside of extension housing).
4)Remove adapter housing bolts.
5) Remove adapter housing by tapping it loose with plastic mallet.
I too have a 2wd tranny here that I need to convert to 4wd one of these days. Came out of a 'manche.