Most likely an axle, as said if there's no noise when rolling it around. Those axles were probably groaning with pain on each lauch at that weight...LOL!

MAKE SURE you check the sprague in the trans (unless it's a bolt in). Don't ask me how I know...

Don't be discouraged by the lack of gain by adding the fresh air. First thing unless the conditions were exact or you have weather station data you'll have to start from stratch. Also keep in mind that changing the torque converter can change the demands of the fuel system depending on the flash at the hit, shift point recovery and overall slippage. With two major changes since your last run you don't know what each one did/didn't do...all the more reason to start fresh with testing the fuel curve.

Now get that rear apart and let's see what you broke!