1998 gmc sierra 2500 7.4

There is a speedometer sensor that may be bad, I looked it up, but couldn't tell you where it is.

Yea that what I guessed but I can't find where I can get one nor where it is

call your local autoparts store or dealer service department. they can usually tell you the location of parts. There should be a code in the computer if the vss is bad. It is also possible that your gauge cluster is at fault. they can be rebuilt. If that turns out to be the case, there are numerous places online to send your cluster. they aren't hard to remove. I have personally used a service in Owensboro Kentucky. chevyspeedo.com. good luck.