Pyth, 11 bytes
Seems rather long.
.cF_tSa0+aF Explanation
Uses xnor's description.
.cF_tSa0+aF Full program. Input: a 2-element list [a,b]. +aF Add |a-b| to the list of inputs. Produces [|a-b|,a,b] a0 Absolute difference with 0 (i.e. absolute value). Vectorizes. tS Sort the list of absolute values and remove the first element. .cF_ Reverse the above and apply nCr to its elements.