There are three springs in the 4WD hub assembly. Two are inside the hub body as part of the works. Both work in pushing the clutch mechanism in and out of engagement. They are a bit more than an inch around, and one is fairly compressed and the other has some space between the coils. Replacements for either of them are going to have to come from a dealership (not likely, but you could check) or out of a junkyard donor. You might try the parts department at
http://isuzu.stcharlesauto.com/ they are very helpful and known for being able to locate hard to find parts. The part number for the compressed spring is 94028150 and the one that is more expanded is 94028149, if they need the group number for both is 6.308
There is a small spring located in the selector switch too. It pushes a little ball outward from the center so that there is a detent at lock and free positions. This one you might be able to find a suitable replacement at a hardware store. The Isuzu/GM part number for this one is 94028148
FYI, these part numbers show the same for 79-82 LUVs. There are a few items that are different in the hubs on the older and newer body styles, but these bits are not one of them.
If you really are missing a part in your hub, I would be concerned that something else might be missing too. If you don't already have it, pick up a Haynes repair manual (most local shops still have them on the shelf for the LUV) and go through all the parts. There is a bunch of stuff in there and it most all needs to be in there for them to work.