Have a 1991 Ford Explorer that I use to tow my boat and the other day I pushed the dash board button to engage the 4wd and I got nada. The little idiot light did not come on and it was clear that the 4wd never attempted to engage. This suggests to me that the problem isn't in the transmission so much as it could be with the electrical connection activating the transmission. Anyone else ever have this problem? Any suggestions on an inexpensive way to test/fix the problem. Not looking forward to taking it into Ford and have them start exercising their adding machine fingers. FWIW, I have a 135k miles on it. The transmission was rebuilt under warranty at approx 45k miles and has been fine since then. I do the normal transmission maintenance and it has not been sounding funky at all... So, anyone have any thoughts?

It's most likely your transfer case motor that is out. That has been a common problem. Check out http://www.glue.umd.edu/~singletn/exp.html and scroll down to transfer case.

Hope that helps.

yup sounds like transfer case problem. completely different animal than the tranny. it could be the tranny motor, or could be the white button on a black box for the module located in the left rear jack compartment

When you push the button, do you hear a clicking noise coming from the rear of your truck? If you hear it, that noise is (I think) the relay trying to activate your dead transfer case motor. In this case you can replace the motor. The dealership wanted quite a bit of money to replace it. I found a used/rebuilt one and installed it myself. Its not difficult. You can easily get to it from the bottom of your truck, its located somewhere in the middle (further back than the transmission).

On the other hand, if you don't even get any clicking, you may want to have it tested to see if there is an electrical problem. The switch could be bad, or a relay somewhere. I don't know what to do in this case.