LA,
The pain and stiffness are why I quit taking Avonex after having been on it for 2-1/2 years. I had very little pain before starting Avonex. I always dealt with the flu-like symptoms but they became bearable after a few months, then the last month I was on Avonex, I had such extreme joint and muscle pain that I could no longer tolerate it. I actually thought it was my MS getting worse as I started having trouble walking normally. Turns out in my case it was the Avonex.
As Cheryl mentioned, you might want to consider a drug holiday to see if your symptoms get better.
I wish you the best of luck and hope you feel better soon.