You can drain the majority of the lube by removing one or two lower cover bolts. Once it stops flowing, replace the bolts. Then add enough kerosene or Diesel fuel to fill it back up to the correct level, jack up a wheel, and run the gears for a few minutes to circulate the solvent.
DO NOT DRIVE THE VEHICLE WITH THE SOLVENT ONBOARD. THE LUBRICATION PROPERTIES ARE SEVERELY COMPROMISED!
After circulating the solvent for a few minutes, remove the cover plate and drain everything. You'll be surprised how clean it will be inside.
Jerry