I had a look at the hitchhiker crab videos I've made over the past year. Three videos of them show the undersides of the crabs in some detail. They all look like they could be female, though they do not look exactly like the female shown in the photo link. I'd venture to say that the crabs we find in our tanks are very young and they may not be showing their male/female traits at such an early age.
See if you can tell me for sure if these are females...
Crab One
Crab Two
Crab Three