I have a 95.5 Toyota Tacoma 3.4 4x4 automatic with ADD. There seems to be a problem with my 4x4 system, engaging on it's own when driving in 2x4.

One day out of the blues, I hear a loud winding, then following by a hard clunk. All in a sudden the 4x4 indicator light is on and I'm stuck in 4x4. This all happened when driving in 4x2 going roughly 45 mph. I quickly pulled over and notice the transfer case lever was still on 2h. It was very stiff turning to the side streets.(4x4 had engaged)

I tired reversing and it would disengage, but would lock itself back to 4x4 in seconds as I'm driving forward. Did a testing on the vacuum, transfercase, transmission and 4x4 system and yet don't know what's the problem..

While on a lift, we tried blowing and manually sucking the vacuum hose and the 4wd did engage and disengage as we shift from 4h to 2h. Check the relay and vacuum switch and is in working condition...

4x4 works with no clunk or grinding or beeping of any kind. Only in 2x4 you will hear the beeping and engaging. Please help