2007 Isuzu NPR TRANSMISSION WON'T SHIFT INTO OVERDRIVE

I have a 2007 Isuzu NPR HD (VIN #JALC4B16177016871) with a 5.2l engine and a Aisin 450-43LE transmission with 91,907 miles that I bought two months ago. Three days ago it started not going into overdrive; it shifts to third gear fine and the overdrive light isn't illuminated which indicated it was in overdrive but past 45mph or so the RPM's stay up (3,100 approx. At 50 mph) and it stays in third. While driving on a level straight road I kept the speed at 50 mph and the rpms at 3,100 and would engage and disengage the overdrive switch on the shifter handle; when I disengaged off the overdrive I could see the rpm's go slightly higher (maybe 100) but then drop down to the previous rpm reading.

I checked the system and found the transmission dark & smelling burnt so I changed replaced the external spin-on filter, drained the pan & replaced the internal filter and filled with fresh Dexron III; the external filter was a NAPA filter so I know it has been replaced at least once before I purchased it. When I pulled the pan the fluid was black and I found three fine strands of what appeared to be aluminum, each of them no wider than a human hair and between 1-4" in length. After servicing the transmission I started the engine and brought the fluid level to an acceptable level to road test; the transmission acted exactly as before, no change whatsoever. I came back and did what limited diagnostics I could do with Mitchell access but couldn't find anything definitive. I have a DTC code reader and had previously checked for codes and found none but checked after the road test and still couldn't find anything. I'm guessing the strands of metal in the pan and the condition of the fluid are telling me that the transmission is toast but don't want to make a thousands of dollars mistake in replacing it if the problem is somewhere else.