Hi everyone,

I have a 1991 4runner (22RE motor) with 4WD and am having a problem with the transmission and/or transfer case.

The symptom: When driving at highway speeds (60+ MPH) in 2WD, a cyclic bumping/pulsing sound emanates from the transmission and/or transfer case. I immediately break to slow down/pull over, and I can feel the bump/pulse as I step on the break. The bump/pulse continues regardless of whether I'm in gear or in neutral, until the car is stopped. The sound slightly resembles driving on uneven pavement or when you have a tire that's about to blow, but that's not what's going on. Sometimes I can drive for hours on the highway and the problem never comes up, sometimes it happens quite often... I've driven long-distance (LA to Phoenix) with the car like this, just pulling over and doing the "quick fix" below if/when the problem comes up until it stops.

The quick fix: after pulling over, I put the truck in 4WD and drive a few feet, then back in 2WD, also in reverse, to "reset" the gearing because it seems that this is where the problem is. I accelerate back to highway speeds and the problem resumes sometimes, less often if I accelerate slowly to 60+ MPH. The problem may or may not come up again. I've also experimented with driving on the highway in 4WD (yes, I know this is bad) and the symptoms never occur, but I don't do it for very long for obvious reasons. It seems that the transmission/transfer case somehow gets "stuck" between 2WD and 4WD, but I don't know enough about automobiles to talk intelligently about it...

What I've done: I explained these symptoms to a mechanic and after he listened to the transfer case with a stethoscope, he was convinced that the drivetrain was bent and that I needed a new one. I had him install a "new" (used/spare/functional) drivetrain for a couple hundred bucks but this didn't fix the problem. Perhaps the symptoms occur less frequently but I'm not sure...

Help!!! Does anyone have any ideas of what might be going on? The truck is great otherwise (even with 250,000+ miles) and I want to fix this!!!

Thanks,

James