It's your driveshaft, but it's not out of balance. You've pushed it past the amount of "bend" it can reasonably handle.
Two ways to fix it:
- Transfer case drop. Basically you put spacers between your frame and the transfer case skid to help straighten out your driveline. Cheap, but not ideal.
- Slip yoke eliminator and double cardan drive shaft. Basically gives you a drive shaft that can handle more bend. This is the right way to fix it, but parts alone are at least $500.