Walker identifies himself as Wonder's nephew and is being held without bail because of another case.
Diaz's bail is set at $95,000. Both are being represented by public defenders, who did not immediately return calls for comment.
Diaz was placed on three years of probation after pleading no contest in February 2011 to possession of marijuana with the intent to sell, court records show.
The case was first reported Friday by celebrity website TMZ.