Does this truck have a multi-part driveshaft? I mean, is it in two pieces, with another U-joint in the middle? If it is, check the pretension on the middle mount. Your mechanic should be able to do this easily, providing he has the spec’s for it (most will).
I don’t think anything is going to fall apart. I still think it is in the engine or something attached to it. It could be something in the torque converter, but I don’t think I would tear that apart for a little vibration.
Anyway, good luck with this.