I did some quick google research and I learned a few things.

In captivity, Mule Deer have been mated to White-tail Deer (in both directions), but less than 50% of the hybrid fawns survived the first few months. Hybrids have been reported in the wild but are disadvantaged because they don't properly inherit survival strategies. Mule Deer bound (all 4 hooves hit the ground at once, called "stotting") to escape predators. Stotting is so specialized that only 100% genetically pure Mule Deer seem able to do it. In captive hybrids, a one-eight White-tail/seven-eighths Mule Deer hybrid has an erratic escape behaviour and would be unlikely to survive to breed. Hybrids do survive on game ranches where both species are kept and predators are controlled by man.