This truck is fairly new to me so I wanted to change the transmission oil. The oil that came out didn't look too bad but attached to my drain plugs magnet is what looks like an interlocking pin or something. It is about 26mm long and one side looks chewed up. What the hell is this thing and how did it get in there?

 

looks like a roller from a caged bearing

 

Roller bearing is a good guess. i also thought it could be a detent pin for a shift fork.
Any trans noise in 1st, 2nd, 3rd, or 5th gear? (quiet in 4th)

 

I was thinking it could be a roller from a bearing but those all have squared offed ends from what I can tell. Based on the size it looks like the longest interlock pin but it does feel normal shifting through the gears. It drives and shifts well. I do have some transmission bearing noise in 3rd when the tranny isn’t fully loaded while coasting but it isn’t extreme.

 

I'd pay attention to any increased noise in transmission, especially any sound that goes away in 4th gear, but is audible in all the others. Mainly 3rd and 5th.