Buford's right in that the 'D' is for 2wd and the 'W' is for 4wd. The difference between the 100 and 150 (same thing for Ford's F-100 and F-150) is that the W-100 was the 'economy' model. Usually low performance, manual trans, economy version. Similar to GM's late '80s and early '90s W/T or work trucks. Just your basic truck.