The Collaborative International Dictionary of English v.0.48:

Axletree \Ax"le*tree`\, n. [Cf. Icel. ["o]xultr?.]
   1. A bar or beam of wood or iron, connecting the opposite
      wheels of a carriage, on the ends of which the wheels
      revolve.
      [1913 Webster]

   2. A spindle or axle of a wheel. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

axletree
    n 1: a dead axle on a carriage or wagon that has terminal
         spindles on which the wheels revolve