Kind of interchangeable- I believe 96-98 are generally the same parts and 99-2002 are roughly the same, with some carryover from 96-98, if that makes some sense.
On running boards- I believe there are subtle differences in the trim only between SR5 and Limited. If you took a 99 SR5 running board and all it's trim it may work fine on your 2000 if it is an SR5 as well. I am guessing? you may have a "base" model that was never equipped with the running boards.
just not sure 100% how is that for vague....? I would try to compare part numbers-