What could cause a low WBC (2.7), low hgb, low and hct, with differential high mono, and high gran?

Virus infection. . Most likely it's a virus infection. If there are no symptoms of a recent infection, a repeat test is important to establish a proper trend. A physical examination is important to look for signs of various illnesses that could lead to these lab results. Remember, labs are not as valuable as a good physical examination.
Altered blood count. There are many conditions, acute and chronic that can alter the blood count and one cannot give you conditions without knowing further details. It is best to see an internist who can sort through this difficulty and discuss this with you to your understanding.
Need more testing. Your blood counts are abnormal. It can be reactive to viral infections or a primary bone marrow or immune related condition. You need to have some tests done including blood levels, viral tigers and may be more specialized testing . Ask to be referred to hematologist.