Several folks have said the battery is the problem. I think 6.24 would start it. It seems to be "breaking down" while cranking. Either the cells in battery are breaking down or the starter is "breaking down"/taking all the voltage. If you want to know if the starter is bad and taking all the voltage, push the truck to start it. (don't forget to turn the key on). If you want to test the battery, take it out and take it to a place that can check it under a machine load. Your problem will be identified doing these things or you will move to other causes. We are guessing right now. You seem to report all other possibilities are new or checked by you. Right now, we have to believe that.

I think I read one time cranking voltage was 2V next time was 4.98V to 5.2V. Did you edit a post?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn