'81 Should be NP119 which is 4wd only. (check the tag to see) The 909 (lockup version of the 904 with SBC pattern) was used. I would look at a 700R4 (4L60, not 4L60E) from an S10 with 4.3L along with the tcase. Not sure on the length, etc, but it will be stronger and have O/D. The Iron Duke used those years is a Chev straight 6 with 2 cyls cut off (also used in Marine Applications and early Chevy Novas)
I assume it doesn't roll when in park? That eliminates the tcase or one of the diffs being gone. If it does roll in park, see if either driveshaft turns when put in gear. There would also be a buzz when put in park like the car was rolling.
The torque converter is the weak point in these. You do know to check fluid level sitting in neutral with it running? If fluid isn't puking out under the car, you could try a can of STP oil treatment in the trans and leave it sit running in drive for a good 5-10 minutes (no fluid pumps in park, which was a flaw in these trans) This is all assuming the shift linkage hasn't come off of the trans?
About 20 years ago I helped my neighbor rebuild the 998 trans in her '79 Spirit (Mexican version with the round opera windows in the rear pillars) I was surprised when I took it apart that I couldn't see anything obvious wrong with it. The high forward clutches had some wear, so I changed them and still had nothing. The torque converter from my Eagle wagon fixed it right up!