Ok, I'll go with Doc on this one. Classic 4th gear switch symptoms.

The switch is on the top right side of the transmission and has a single red wire.

As Doc inidcated, you may not have to move the tranny depending upon how limber you are and the type of socket, swivel, extension and ratchet you have.

If you have trouble getting at the switch, a trick that I use is as follows:

Support trans with a hydraulic jack. Remove the transmission crossmember support bolts on the LEFT side of vehicle and REINSTALL...THREE full turns.

Remove right side bolts all the way. Slowly lower trans with jack until the support hits the left side bolts...then slowly lower the trans further which will roll the tranny over to the right until you can get on the switch with your socket. Doesn't have to be far.

Change the switch.

As you jack the tranny back up, place a long drift or Phillips head screwdriver through one set of right bolt holes to line up the cross member as it moves back up. Install the bolts and tighten all the support bolts to 37 ft lbs.

Drive happy.

Duane