[Steeveedee]

Sounds like the vacuum modulator or the line to it. Pull the modulator vacuum hose up on the manifold and see if there is ATF in it. Since it is a 4X4, it might be easier to just pull the line at the trans. The vacuum modulator is on the passenger's side of the trans. If you get fluid out of it, it's bad. It might be bad anyway, but fluid in the vacuum line is a certainty that it is toast. And they can go bad, just like that, too.