I sold a residential property in Arizona 1 1/2 years ago that I "hold"
the mortgage and it is being serviced through a title company.  My
buyers are now in default and I have given them notice which will
"expire" on March 21st.  At that time, I can begin a trustee sale.  My
question is:  can I just re-claim and move back into the property as
opposed to selling the property?

Thank you for any guidance you can give me.

you might get a more accurate and reliable answer at about $100 since
it would cost you about $400 to get a lawyer to give you a REAL
answer. Just a suggestion.

Please read the disclaimer below.  This is not professional or legal advice!

I've been through a very similar situation.  Although you may feel
that the property is somehow still yours (mortgage and all), the
people bought it and are now the "owners".  A trustee sale is a new
business transaction. If the property could be sold now for more than
what is owed to you (which you would receive), the balance would go to
those people, that is why there has to be a sale (I believe - see
disclaimer).
That may seem sort of unfair  - their making a profit while defaulting
on their mortgage payments - but since this is a new and separate
transaction, if you get full payment for the sale of the property a
year and a half ago, your interests from your sale have been
satisfied.
IF you are interested in regaining ownership of the property, you have
to buy it back.  But since you don't have to pay yourself to settle
the mortgage, you are in a better position than someone else.
See again the disclaimer and siliconsamurai-ga 's comment.
There are probably also books on the subject.