50mph Screach

I am not aware there was ever a “long” 16 tooth T90-C input shaft, OEM or aftermarket.
I agree with durangotang with a screech is likely a severely worn intermediate shaft/needle bearings/thrust washers in the transfercase. When worn badly, the actual gear tries to twist between the case sides under different load conditions. This is why the sound changes whether on throttle, off throttle, or running steady.
-Donny

 

Update: so, as it turns out....I have a T14 in my jeep. Not a t86. Must've been swapped in by a previous owner. I bought a rebuild kit for it...but now I'm starting to wonder if maybe it is indeed an intermediate shaft in the transfer case.