That source states that In captivity, Mule Deer have been mated to White-tail Deer. Both male Mule Deer/female White-tail and male White-tail/female Mule deer matings have produced hybrids. Less than 50% of the hybrid fawns survived their first few months.

The source states nothing regarding the fertility of such offspring other than an off hand quote from Darwin which is completely taken out of context and has no bearing on the species being discussed. If what this source states is true, it appears somewhat of a moot point as they probably will not survive long enough to potentially breed and then may suffer from other sources of isolation like behavioral differences.

As far as Wiki goes it is a good source of general information but an obscure question like this is best addressed using peer-reviewed literature.

I'll continue tolook for a fertility study done on hybrids, I'm certain there has been done something done it's just a matter of finding it.