I have a 7 yr. old Yorchi (yorkshire terrier/chihuahua)that is very intelligent and well behaved EXCEPT he has this problem with barking & nipping at people's heels when they LEAVE the house at the front door! I could understand barking when someone is at the door coming in, but leaving? Mostly I notice that if someone is wearing tennis shoes, and they are male, it's worse. He nips and draws blood sometimes and it's not strangers, it can be anyone even people he loves. It's not always, but I can't figure out what triggers it. Please help.

This is pretty classic of small fear-aggressive dogs. Every time he runs and bites a person leaving, what happens? The person leaves the house! So he tries it again. Same thing! Success! So the dog goes after the less threatening (walking away vs. very threatening walking towards the dog) body language and it works EVERY time!
Ideally you should work with a behaviorist in person. Your vet should be able to refer you to the nearest veterinary behaviorist or behaviorist with a PhD. If you need help locating one giving your general location would help.
Here are some books on how to help fear-aggressive dogs:
http://www.dogwise.com/itemdetails.cfm?ID=DTB586
http://www.dogwise.com/itemdetails.cfm?ID=DTB878
http://www.dogwise.com/ItemDetails.cfm?ID=DTB799

Gosh, that makes so much sense. Thank you sooo much!
Theresa