When my truck is in neutral it's fine but when I have it in gear and accelerate it clacks and kinda squeaks bad. It comes from under the truck and behind the cab I think. Any thoughts what this might be?? Universal joints, transmission, rear end??

 

Sounds like u-joints. You can usually feel when they go bad cause they will have play in them, if not see if they have grease fittings. If they do and the squeaks goes away and clangs a little less than that is an indication they need to be replaced. And if your able to lift your rear end up, spin the wheels and see how they feels, turn the drive shaft and see how it feels.

 

U-joints

 

Check u-joint and check center support joint if you have a 2 piece. Replace them ALL while you are there. Joints are cheap.