Weird sound during acceleration

We all know what squeaky leaf springs sound like... This is similar sounding but only happens during acceleration. It's almost like a squeaky wheel. It sounds like it's coming from the rear when I'm driving. It who really knows. So far as I can tell it happens at any speed when I accelerate, but not when I'm coasting. I assumed it was just dirty but I hit everything under my truck with the hose and it's still there.

And to add a wrinkle... I currently have a bed full of dirt, and in addition to the squeak under acceleration, I also get a weird vibe. It feels kinda like an out of balance tire, but again this goes away when I am not in the throttle (even at highway speeds). I do need new tired and plan on getting those in the next few weeks but I can't see how tires would only cause issues during acceleration.

Any ideas? I don't normally have a load in my truck at all so I have no prior experience under weight. It's been squeaky for a while and I can't think of anything specific that could have happened.

Thanks!

EDIT - so i got a bunch of the weight out and the vibration is reduced a lot. it seems more like a shudder during acceleration (still kinda like an out of balance tire) but it isnt there when coasting at the same speeds

 

Ur Sig pic shows u like mud..... U sure its not a pulley?

 

It feels like its in the rear. I have repaced the ones in the front. Are there more?

 

The vibrations seem to be the worst between 30-40mph. Or 2nd and 3rd gear

 

Mid-shaft bearing? Drive shaft U-joints lubed?

 

Same with my truck. Squeaky nose on acceleration and vibrations at 30 to 40 mph. Rear U joint rusted solid, middle U joint almost seized. Might as well replace center bearing too.
Fixed my vibes and noise.