Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
If you are hearing a fast whirring sound then your issue is the starter still. If the flywheel were damaged (which is painfully unlikely anyway) it would not make a whirring sound at all... it would make an awful grinding sound. Even at that, the flywheel could easily be inspected (And should have been already since the starter has been out three times)... with the starter removed there is a perfectly clear view of the flywheel contact area, and the engine can be rotated by hand at the crank bolt to verify the condition around the perimeter with no need to remove the gearbox "just to see".
What I would be doing here is checking your cable condition between the battery and starter to make sure there isn't anything unusual going on with the power supply (weakened cable from oil saturation, corrosion etc). If the cable is in good shape I would be getting a new starter for the truck and calling it a day. I can appreciate that several people have attempted to fix the existing starter, but it has to be remembered that starter overhauls usually result in new starter contacts, sometimes an over run clutch or a main drive armature, but even at that those instances are rare. Most people put new contacts in and clean the armature and call it good. That is fine if that what it needs, but you are more likely to have an internal switch issue, a pinion lever issue, or something like that. And in those cases (1) no one stocks those sort of parts so they would be very unlikely to have been replaced as opposed to a standard armature/contact overhaul and (2) the cost of the parts to do a proper full overhaul exceed the cost of a new starter in most cases so the shop can't really make any money if they were to actually replace everything properly.
If the flywheel is still a concern, again pull the starter out and inspect through the starter hole while rotating the engine by hand.... by your description though if you don't have anything obvious like a power drop to the starter you just need to get a new starter in there, not try to fix the bad one any more.