'86 Sierra won't start...

[The_Animal]

I have a 1986 GMC Sierra 1/2 tonne p/u that has a starting problem. The problem is as follows:

The truck cranks slowly when I turn the ignition, however it will start only occasionally. The rest of the time it doesn't even start. My gut feeling is that the starter solenoid is gone. I don't have the money to take it into the garage and get them to replace my starter, and I feel that I have to get under the hood myself and do it to save money.

My question is what systematic elimination of the list of potential problems should I do to try to ascertain what exactly is the problem. As far as I know, the battery charge indicator on the truck reads full-charge, when I turn the ignition, though it jumps back to ZERO. Which can only mean to me that when I turn the ignition, the starter is taking up too much juice indicating a starter problem.

IF the alternator was the problem, then the battery would be completely drained, wouldn't it?

I've checked the battery terminals, they appear to be clean and un-corroded.

So, other than the solenoid, what other problems other than a worn out starter motor could cause the symptoms that I've just described? And how can I rebuild the starter motor, if it is worn out. My Haynes manual only describes the solenoid replacement.