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.