I just sucked it out into a vaccum brake-bleeder bottle when changing the filter. That doesn't get everything in the gearbox, but since the fluid was clean, it is likely good-enough. I don't recall a drain plug on the gearbox. I filled with ATF+4 which Chrysler uses and a TSB recommended when servicing all older power steering, presumably back to the 1960's which had a similar Pitman-arm gearbox. They amusingly sell the same fluid in a "power steeing" jug at dealers at much higher price than Autozone. Many power steering systems spec Dexron III ATF fluid (GM). ATF+4 is fully-synthetic and pricier, so might be better, plus I keep jugs for my Mopar vehicles. You could also go to a M-B dealer and pay whatever for fluid. With ATF, you will have to live with red dye, though might help in finding leaks.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans