I'm sorry to hear that your having such bad luck with these re-manufactured starters, I want you to know that you're not alone, in my there are many companies we just won't buy starters because the failure rate was simply to high, even when buying re-manufactured starters direct from BMW over 50% of them failed within the first week of being in the car.
It sounds like you're doing everything right, you've replaced the bad starter, taking the time to shim it correctly (as these trucks were not shimmed from the factory many people overlook the need for shims as the thrust bearing wears), replaced the solenoid in case it was sticking or had bad contacts, and you even went so far as to replace the flywheel to make sure all the teeth the starter engages are in good condition.
My recommendation to you would be to buy a new starter from Remy, this is the only company my shop will buy starters and alternators from and one of the nice things about them is that they offer both re-manufactured and new starters for your truck, you really can't go wrong with either one but for $50 vs $77 I think I'd go with the new one myself.
REMY 97132 (New) $77
REMY 25402 (Re-manufactured) $50 + $30 core
MOTORCRAFT SA734BRM (Re-manufactured) $67 + $75 core
My pricing is from rockauto.com I've found them to be an excellent parts supplier with really competitive prices.
I hope I've been able to help you with your concern, if you have any other questions or if you would like more information on your current question please feel free to ask.