Hey everyone, two different problems that I suspect are related. ('97 4Runner, 5spd, 140XXX miles)

1) At any speed above 50mph, when the truck has no positive or negative pull (like coasting in gear or in nuetral), the truck rumbles, sometimes badly. It's not a wheel issue, because they've been balanced/rotated and no difference. The truck doesn't shake, it's just a small vibration with a big noise.

2) Again, with no negative or positive power, my 3rd and 4th gears can slip out into nuetral all on their own. Sometimes the other gears as well, but especially 3rd and 4th gears.

Also, i've noticed that my 3rd and 4th gears don't physically shift as high or low as any other gear.

What do you guys think? Could the tranny problems be causing rumbling? I took the truck to Toyota and they couldn't find anything wrong..
Thanks in advance to everyone.

Grab 4 jackstands and a friend, get the truck in the air, and 'drive' it to verifiy your noise. Truck doesn't need to be too far off the ground, just enough for the tires to clear.

When's the last time the gear oil was changed? Start there anyway.

I dont know if this applies in your case, but i had a similar problem, but i could feel the vibration if i kept my foot barely touching the gas pedal... turned out to be bad rear end gears and it happened when there was no positive or negative pressure on them (bad gear lash almost). Replaced the rear end gears and never had a problem since (70,000 miles when replaced... 110,000 currently)