At this age of the trucks, the thing I'm concerned with on the trail (or street for that matter) is the distributor. We seem to be in the age of common failures within the distributor (module, pickup coil, etc) and it's harder to replace the pickup than the whole distributor.
Coil itself would be another I'd not mind a spare of.
Of course, the only spare I carry is the semi-serpentine belt. I've been left stranded by the pickup coil, which is why if you haven't replaced the internals of the distributor already, I think it's a good idea to have a complete spare distributor that is known good (or new).