As I keep repeatedly saying "NO, not 1991 or 1991.5!"
It was not until 1992 (on Diesel trucks) that the speed sensor was added to initiate the charging circuit.

There are fables out there saying that there were "a few very late 1991.5 trucks" that had it, but I have yet to find any mention of it in TSBs, or service manuals, or otherwise.

Mark.

 

Welp boys and girls I found the problem! It was the ECM, luckily I kept the old one when I replaced it. I plugged the old ECM and voila the tach started working and the a/c. Funny thing is I didn't even need to buy a replacement ECM because the old one was just fine, what threw me off thinking I needed a new ECM is because I have owned several OBD1 Dodge trucks and they were notorious for the ECM failing and not allowing the vehicle to go into overdrive. So naturally I went with the 200 dollar fix try first instead of tearing the tranny down. Come to find out the main problem was when the morons I paid to rebuild the tranny rebuilt it, they used the old torque converter and apparently one of the dropped it bending the shaft on the converter, I noticed it when I put it back together and I couldn't turn the flywheel freely to line the bolts up with the holes on the converter, and it was blowing transmission fluid past the front seal. So needless to say the nightmare is finally over. Thank you guys for all of your help!

 

Not very nice to NOT directly answer the questions you are being posed!

Mark.