It cold be your speed sensor mine did that same
I Have A 97 Gmc Sierra Z71 4x4 The Transmission Will Not Shift Into 3rd Gear The Trans Has Just Been Rebuilt And Has Been Working Fine The Fluid Level Is Full And I Have Adjusted The Neutral Safety Switch Still I Am Having Problems Please Help?
You need to adjust passing gear cable. There is a plunger you push in.If you have a local mechanic he should do it for free it is easy.