What year truck are you dealing with? 6 or 12 volt battery? Have you checked the battery terminal voltage while you're trying to run the starter? If you've got a 6 volt starter with a 12 volt solenoid installed, the symptoms you're describing are a possibility. The starter armature should not turn at all until the gears are engaged unless you've got a very bad linkage problem between the solenoid, the drive gear, and the disc switch at the back of the solenoid. I think it's time to pull it all back apart, inspect everything again, and bench-test the starter after you reassemble it before putting it back on. With the starter motor clamped in a vise, it's possible to check out all the functions before you reinstall it.
Jerry
