"Freak" and "freak out" mean the same thing.
Here is the definition for "freak," listed as both a transitive and intransitive verb and described as slang, from the American Heritage Dictionary*:
1. To experience or cause to experience frightening hallucinations or feelings of paranoia, especially as a result of taking a drug. Often used with out.
2. To behave or cause to behave irrationally and uncontrollably. Often used with out.
3. To become or cause to become greatly excited or upset. Often used with out.
The Collins COBUILD** gives a similar explanation with a comment: Freak out means the same as freak.
There are some other slang expressions which seem to evolve from two-word verbs into one-word verbs with the same meaning. For example:
"Let's hang together" means the same as "Let's hang out together."
"I caved" means the same as "I caved in."
* The American Heritage Dictionary of the English Language, Fourth Edition Houghton Mifflin Company. 2003
**The Collins COBUILD English Dictionary. Harper Collins. 1995