The Collaborative International Dictionary of English v.0.48:

Unburden \Un*bur"den\, v. t. [1st pref. un- + burden.]
   1. To relieve from a burden.
      [1913 Webster]

   2. To throw off, as a burden; to unload.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unburden
    v 1: free or relieve (someone) of a burden
    2: take the burden off; remove the burden from; "unburden the
       donkey" [syn: unburden, disburden] [ant: burden,
       burthen, weight, weight down]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "unburden":
   alleviate, be light, break bulk, clear, disburden, discharge,
   disembarrass, disencumber, dump, ease, have little weight,
   kick the beam, lighten, lose, make light, make lighter, off-load,
   reduce weight, relieve, rid, set at ease, shake off, throw off,
   unballast, unfreight, unlade, unload, unpack, unship,
   weigh lightly