Well, I am going to guess that Toyota want you to use either a GL4 or GL5 70W90 Gearlube. If it has to be available at Autozone or Walmart I would say M1 or Costal Synthetic.
As far as non-synthetics go I have used Castrol,Havoline,Valvoline,Walmart Brand, Bulk pail from Sam's club and never felt a difference in shift quality or performance in the rear end on Toyota trucks!
Seeing how you have no intensions of changeing it ever again I would definately spring for the synthetic gear lube!