Toyota Repair: 99 Tacoma knocking/clacking noise, splined shaft, punch mark

Question
I have a 99 Tacoma PreRunner, 2 wheel-drive w/ the 2.7L engine.  When I accelerate in the range from 0 - 30 mph, I hear a bad knocking/clacking noise(metal hitting on metal) underneath the truck.  The knocking noise only happens when you accelerate forward or go in reverse slowly, not when the engine is idle.    The noise has been getting progressively worse over the past two weeks or so.  Also, after 30 mph, the entire truck vibrates so bad it feels like its going to fall apart.  I dont think its anything to do with the front end bc the steering wheel does not shake/move when the vibrations start.   Knocking noise sounds like its coming from the drivetrain, rear end, or the transmission.  Know a mechanic who thought it might be one of the u-joints on the driveshaft throwing the driveshaft out of balance. I pulled the driveshaft and all of the u-joints are tight,there is no play/slack in them at all.  The weights on the driveshaft are still there. After removing the driveshaft, you can move the rear differential by hand about a half inch each way where the driveshaft connects; it seems like that is too much play?  Is that normal to be able to move the rear differential slightly?  Im sure you've heard something like this before from this model/year of vehicle and Im truely at a loss of what it could be.  Thanks for your time and attention.  ~Brad

Answer
If the driveshaft flange on the differential can be moved that much there is definetely a problem there,check to see if the pinion nut has come loose, you may be able to tighten it back up until there is resistance then tighten it a little more until the punch mark on the nut lines up with the notch on the splined shaft, tightening it too much will destroy the pinion bearing, if it's too loose there should be some noise.