Providing your cables are good, battery good, and connections are clean, and wires at the starter are tight and not pinched or frayed at their connectors ....I'd say remove & take the starter apart. Have a look inside. Usually the problems are obvious. They're easy and fun to rebuild. A couple possibilities come to mind:
1) Pitted solenoid contacts: If the starter is original, or really old, the solenoid on the starter has large electrical contacts inside, which when the solenoid plunger gets "pulled back" prior to the motor engaging, when the solenoid plunger bottoms out, the contacts on the plunger bridges the gap between electrical contacts and allows current to flow between the big battery + cable and the starter motor. If you were to cut open an old solenoid, those heavy copper contacts become real pitted and eroded away after many yrs. That pitting or low contact area can cause reduced current flow and a slow starter. Make it spin slower.
2) Worn Brushes: Brushes are the electrical contact from the starter body to the spinning motor shaft. They ride on the shaft Commutator. They are high in carbon and wear due to sparking, etc which is normal. Brushes are a certain length and eventually they wear out just as pencil lead eventually wears out. They can be replaced pretty inexpensively. If your starter has age to it, this or #1 (or both) are likely your problems. Worn brushes eventually won't have enough material to contact the commutator and eventually won't work at all.
3) Flooded with oil. And engine oil leak seems to always get onto and inside engine starters. They can get oil-logged (like water-logged). And that can cause a slow starter. If your truck is dry and clean underneath, this won't be your issue.
4) Corroded or damaged cable/bar: On the starter motor itself there is usually a metal bar or often a short braided cable ...or a couple inches which connects current from the solenoid to the actual starter motor. These cables can corrode over time. Especially in the salty or rainy northern states. Idk about AZ.
5) Heat-damaged field coils: This is a problem on starters, esp starters located close to the exhaust.
6) Poor Ground/s: Ensure the engine and body grounds are good. It doesn't hurt to run a jumper cable from the battery neg to the starter body (not electrical contacts).
7) Other: There are a few other things which can go wrong in a starter, like worn bushings, failed part, things like that. A 'Bendix' (one-way starter drive) can fail, but that makes a high-pitched 'Ziiiiiiing!!' noise, not your current problem.
There you go. You can buy a new or rebuilt starter, but I encourage you to remove and disassemble your vehicle's starter. Understanding a starter or Alternator is good knowledge in case you ever need to do an unexpected field repair. There are places on the internet including ebay/Amazon and specialized places which sell all sorts of starter parts, and of course all sorts of how-to videos. You might save a few bucks... but overall, it's fun to do and good to know. Let us know what it turns out to be...
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