The starting issues are among the most common on these older vehicles and are rarely the starter. The problem seems to be with older wiring and not being able to carry enough current to complete the circuit. There is a mod that bypasses some of the long circuit and allows power to be delivered to the starter directly by tripping a relay that requires only minimal voltage input. It used to be posted in the How to's section I believe about 10 years ago. Yeah, some of us have been around that long :{
I think it mightve been posted up by 88pathoffroad since he did the headlight mod. ( same issue with old wiring making headlights dimmer over time) By using relays can transferr power to lights directly from alty or battery giving brighter lights)
An easy test for your starter is to jump it directly from the battery. If it cranks it aint the starter and likelly you will benefit from the relay mod. Search on here for it. If you cant find it let me know and I will take pictures of how it looks in my truck. I couldn't find it when I just searched ( there are 250+ pages of starter issues) Perhaps one of the long time members or mods has it archived and can resurrect it and pin it to the How Tos where it rightly belongs.
Mr. Reverse has it right as far as testing. The relay mod involves dividing those wires and installing the relay which will trip with much less voltage. Actually, now that i think about it, I think the mod was posted by Mr. Reverse !