I posted a while back about how immediately after I got the torque converter installed (immediately as in on the way home from the shop) something would vibrate ever so slightly under the truck. I got a lot of input about different possibilities (heat shields, etc.) and removed those, but the problem is still there.

Except it's getting a LOT worse. Here's what it does:

The vibration ALWAYS starts when I gently ease on the throttle. Never below like 30 mph or pulling out of the driveway or anything - just mainly if I'm out driving normally and give it just a little bit of pedal to maintain my speed - it will make a very loud vibrating/rattling sound. It never seems to do it in 1st gear - not sure about second. But definitely in 4th and pretty sure it does it in 3rd too. Since it's only in certain gears, could it be clutch packs in the tranny? Or maybe it's a flaw in the converter? I'm lost here.

Any help would be appreciated.