Hopfully easy questions for you chevy guys. Pardon my lack of info.

Im looking for a junk yard frame for my 4x4 project and they want me to tell them a year and make of truck to look for. So on newer, as in 90's 1500 series I notice the long, low straite section of the frame hanging down from front to back. This is a little different from the older chevy trucks and is why this frame is perfect for my project.
My thought is that the older of truck I find in the pile, the cheaper it might be. So what year did they change the frame rails to look like that? Im guessing somewhere in the late 80's early 90's. And what models apply to this frame Im looking for. My other question is on frame simularity between the 2wd and 4 wd versions in the years and models that of trucks im looking for. Im guessing that maybe a heavy duty version of a 1500 series might have a different frame.
Does anybody know this stuff

 

The pick-up frame changed with the body change in 1988 and remained the same till mid-1999. As far as the difference between 2 and 4 wheel drive I am not sure.