Slightly confused on this issue.

Are LD and HD trucks based on model designations like, Dakota and 1500 are LD, 2500 and 3500 are HD or does this have more to do with various options available on any truck like Engine/transmission/axle ratios or is it more suspension related?

More importantly,
Were gas 5.9s available in both LD and HD trucks?
Were both available in either 2WD or 4WD?
Were 5 speeds available in both designations and were there gear ratios between the two different?
Is the engine mapping different from LD to HD and if so, which PCM is more suitable for "performance" applications?

I'm 99.9% sure the gas 5.9 was available in both 1/2, 3/4, and 1 ton trucks
both were available in 2wd and 4wd

im not sure about the gear ratios