Its a part that may or may not appear on every truck due to the speedo gears in the transmission (or tcase) and choice of axle ratio. If your speedo runs too fast or to slow, it stands to reason this missing part could be a cause.

 

I have had work done as asked. But nothing to do with transmission or speedo. I guess it’s possible someone stole the part while they had it. The nut was finger loose I noticed upon removal. Although nothing looked touched so who knows perhaps a coincidence?