Hey,
I have a 93 4runner, 3.0 auto and the 4wd light is flickering on and off and there is a loud thud once in a while from the front end.

This is happening on the highway in 2wd. That light gets real annoying, click, click, click, click, click, click, click, click, click, click, click...
but I am more worried about breaking something.

any ideas?

 

I'm not super familiar with the 4WD wiring schematic but I'm pretty sure there is a relay in there that actuates the system. Sounds like your 4WD is indeed trying to engage. This could be bad at highway speeds. The book says to keep it under 50 but even that is pretty harsh on it. I would disconnect the plug on your front diff to disable it until you figure out what's going on.

 

I think the plug is just for the system to verify that it is actuated or for the indicator light.

Vacuum actuates the front diff. You could disconnect and plug the line on the firewall or at the diff.

 

http://www.off-road.com/trucks4x4/ar....jsp?id=399612

Best link I know of when troubleshooting Toyota's ADD.