I have a 97 c2500 I just bought. The tranny wouldnt shift out at all so I towed it home.

I changed the filter and fluid and it shifts fine now through 3rd, but when it gets to 4th(od), it shifts into OD then starts sliping until I manually put it back in 3rd.

Is this tranny shot or could it be something mechanical that controls the OD?

Thank you in advance!

Beuller?