D 100/150 trucks are 1/2 ton 2wd.
W 100/150 trucks are 1/2 ton 4x4.
D200/250 are 3/4 ton 2x4
W200/250 are 3/4 ton 4x4
and so on.

The difference between a 100 and 150 is simply the year of manufacture. Ford became an F150, Dodge followed suit. GM jumped from 10 to 1500, and Dodge followed that (later).