Depends on many factors. If you do a fellowship in California you have a very high chance of getting a job in SoCali. If you don't do your fellowship in California, then things will depend on the type of fellowship that you do (IR versus Nucs) and the institution that you do your fellowship (MGH or Hopkins versus a small program). Generally speaking it is not easy, but doable and easier than what you may think.
Don't assume because you go to MGH or MIR you are guaranteed a fellowship at UCLA or Stanford. Let's say you want to do a certain fellowship esp something more competitive like IR or MSK. Many of these programs fill their spots internally. For example, Stanford fills most of its MSK spots internally. There are times that they even don't accept outside applicants.
On a tangent, in the worst case scenario if you don't find a job right after fellowship, get a job in another place and keep your eyes open at your desirable area. You will find a job sooner than later. A friend of mine was looking into Boston market and couldn't find a job a first. He got his first job in Florida and later moved to Boston as soon as he could find something. Don't forget that most groups will choose a radiologist with 2-3 years of real life experience over a fresh graduate of UCSF or MGH.
