Last year, we lost a dear friend, beloved and all around amazing guy. He did not die from his underlying illness, he died from complications of long term steroid use. I do not recommend long term steroid use and I don't know many doctors who do.
Plaquenil is an anti-malarial which is also used to treat other disorders/diseases such as discoid or systemic lupus erythematosus and rheumatoid arthritis in patients whose symptoms have not improved with other treatments.
Having gone over the National Institute of Medicine information http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000171/ the side effects you are experiencing are not typical among known side effects of Plaquenil (Hydroxychloroquine). Usually, it will either help with the symptoms of Sjogrens or it won't. It wouldn't hurt to contact a pharmacist and ask about possible drug interactions with steroids. There are 205 drugs known to have interactions with Plaquenil and Prednisone is one of them. If your pharmacist and/or rheumatologist didn't catch that, and you didn't read the package insert, you should probably talk to a pharmacist to determine whether or not you should continue using Plaquenil if you're using Prednisone still.
You might be interested to know that the side effects you are reporting are not consistent with Plaquenil but are known side-effects of long term Prednisone use.
Definitely talk to your doctors about this. Prednisons is addictive and can have some pretty serious risks when used longer than seven (7) days.