Your are correct, D21 signifies a Heavy Duty 4 speed trans.
These are all the codes for axle packages:
A31 - 3.91:1 HP axle package
A32 - 4.10:1 dana 60 performance axle package
A33 - 3.54:1 dana 60 track package
A34 - 4.10:1 dana 60 super track package
A36 - 3.55:1 HP axle package
If you have one of these, it will be on your fender tags.
... and most of the photos of tags I have here typically show it on the fourth row...
But not every tag will have an axle code...