Recently I put an M46 trans into my '80 which originally had an M45. I'm trying to complete some other work and I haven't driven the car yet but I've been a little concerned about speedometer accuracy.
I found an article at http://www-unix.oit.umass.edu/~tcroy/articles/rearends.htm which identifies which speedo driven (small) gear (at the trans or in this case o/d unit) is used with which differential.
According to this info I'm okay. My car has a 3.91:1 diff and the gear in the o/d has 20 teeth.
What puzzles me is that the '84 donor car should not have had a 3.91 rear end. At least according to the owner's book and the article cited above. Which does not mean it couldn't have had. It's too late to look because the car is gone forever.
For what it's worth, the M45 that came out of the '80 also has a driven gear with with 20 teeth. So if the drive gear (on the trans or o/d output shaft) is the same in both M45 and M46 then I'm wasting my time pondering this little mystery.
I guess I'll find out when I get the car on the road (hopefully, soon).

But, does anyone know if an '84 DL would have had the 3.91 from the factory?

And is the drive gear (not the driven gear) is the same on both M45 and M46 transmissions?
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'80 DL 2 dr