The Collaborative International Dictionary of English v.0.48:

Rack \Rack\, v. i. [imp. & p. p. Racked (r[a^]kt); p. pr. &
   vb. n. Racking.] [See Rack that which stretches, or
   Rock, v.]
   To amble fast, causing a rocking or swaying motion of the
   body; to pace; -- said of a horse. --Fuller.
   [1913 Webster]