If you look more carefully, you may find the nut still there, sunk into the rubber bushing. A common problem on these trucks was that the top bushing cup would wear on the sharp points of the nut and break, eventually pushing over the nut and falling off. This seemed to only occur on the 2 wheel drive models, which looks like what you have.
In the day, there were no aftermarket stabilzer link kits available for the Nissan Frontiers and Hardbodies, but that may have changed. You can always checks with your local parts store or search online. From Nissan, each part of the link came seperate and is expensive to replace the entire link kit (compared to the cost of a typical link kit for other vehicles). In my notes, I have the following part numbers, which may be superceded by now:
Nissan D21-D22 P/U, 2WD, Sway Bar Link parts:
Rod-54618-01G00 (1 per side)
Bushing Cups-54622-0B000 (4 per side)
Rubber Bushings-54619-8B400 (4 per side)
Nuts- 08912-7401A (2 per side)
Yeah, Pep Boys didn't carry it. I called my Nissan dealership and they want $25 for the rod and $6.50 for each bushing. Would it be wrong to just replace the top bushing cups and bushings? My dealership also doesn't have any manual transmission fluid. I tried two different dealerships. Can you recommend a good manual transmission fluid?