Yes the current SD cab goes back to the intro of the 97 F150, there was no 1998 F350. The last of the OBS 97 HD's (Who wouldn't kill for the opportunity these days to buy a new 97 Powerstroke!) were on the lot at the same time as the 99 SD's.
I remember reading that the though process was that the F150 and SD were trying to target two completely different markets with the 150 going more care like and the SD going for traditional buyers.