If the rest of the engine is sound, then dropping $5000 for a truck and replacing the bull gear is a good deal. Detroit recommends replacing the bull gear after 800,000 anyway. And there are rebuilt bull gears on the market that are more reasonable than a brand new one, and come with warranties also. One can generally tell where the metal of coming from, if they know what the metal is, but in the final story, no way to know for sure without something getting opened up, and more than likely, at least a basic inflame might be necessary, if not a total rebuild.
when my bull gear bearing was leaving metal in the oil, it could not be diagnosed by knowing what type of metal is was cause there are more metal parts in a engine that have the same metal type, like main bearing & bull gear bearing.
It turned out is the air compressor. Put new air compressor, changed the oil, drove 3000 miles, changed the oil again - no metal particles in the oil.
How would it be worth it 5000 plus 20000 for in inframe on a freightliner columbia for 25000 you can get a pete or kw or the guy selling an 2007 international 9900 for 22000 on here with a cummins and a 13 speed.
No offence, but your prices are little off, for $20 000 - I can get a factory rebuilt engine by Detroit with 300 000 miles warranty. For $8-10 000 I can get a in frame in local shop. Don't get me start it on Cummins EGR problems and head gasket oil leaking on cylinder N0 6. The best engine Cummins ever built is N14 RED TOP, but it is history.....