Take the rear cover plate off and look for damaged gears, metal filings, etc. Be sure to have a drain pan ready as you remove the bolts- - - -the remaining lube, if any, will be a nasty, stinky mess. Assuming you don't find expensive bits and pieces in the bottom of the housing, flush the housing well with mineral spirits or Diesel fuel (I use a hand-pumped garden sprayer) reinstall the plate, and refill to the level of the fill plug with fresh 90 weight lube. Things will either improve or not. Either way, all you're out of pocket so far is an hour's worth of work and a couple of bottles of 90 weight. It's also possible he's got a bad axle bearing on the side that's stiff to turn.
Jerry
