I wonder if I’m at risk with my 21 year old SUV.
I think you’re ok. This drive by wire business is relatively recent.
Probably. If you have the OBDII interface, you're at risk for some sort of hack. Now, the amount of things controlled by your computer is likely to be fewer than today's cars, but you still probably have a computer (my 1989 Mustang had one, and the 1994 Chevy Silverado 1-ton dually did, as well).
So they could affect engine speed/operation/shifting (if automatic). Unless you had ABS, or shift-by-wire 4-wheel-drive, that would likely be unaffected, but I know that GM's 4x4s in the early/mid 1990s had shift levers for their transfer cases in trucks/SUVs, but it's my understanding that they were fully fly-by wire and the shift lever was just there to make you think they were mechanically linked. My old boss had one go out, and he was surprised to learn that (as was I).