05 sierra Stuck in 2nd

[ColoradoKid512]

05 Sierra 1500 WT 4wd: 3rd gear not working, shifts from Rev, 1st, 2nd, n vice versa but won't shift into 3rd. Fluids are good, fuses are good, checking wiring now but I'm looking for the most common causes of such a problem. Tranny shop wanted to rebuild and charge more than I paid for the truck.
320k+ miles but new (jasper) tyranny. Thoughts?

 

[TollKeeper]

Hey there neighbor! Follow Coloradian here!

Since you have the 4L60, it could be a shift solenoid, which were known, and prone to failure..

But before you throw the parts cannon at it, I will let someone who knows more about these trannys than me chime in!

 

[Mooseman]

Is the CEL on? Any codes?