Rob,
Can't speak to NWA's DC9 fleet but would be surprised if it were any different from anyone else. The DC9 ( not MD80 or B717 ) is an old airplane with perhaps a limited life given the state of the US airline industry, but at least they're paid for !!
The DC9's I flew had 2 VOR's, 2 DME's, and one ADF. DME was most often used to ID intersections as that info is usually published. Absent that, VOR cross radials were used.
It may seem a bit antiquated compard to GPS, FMS, EFIS, INS, etc., but the system still works well.
Pehaps out there somewhere is a DC9 with a more advanced nav system but I've not heard of such a thing...so far.