Is there any recourse when an old employer is harassing me and threatening to send me to collections for their mistake?

Apparently, from 2010 to the beginning of 2013, my job screwed up and did not take out social security. They found the error when I went from part time to full time. For over two years, I have been waiting for a correct calculation. I got laid off in December 2014 and all of a sudden, I am getting letters with different amounts as to what I owe, letters demanding payments, letters saying I will go to collections etc. I have asked several times for all copies of my paycheck stubs from the duration this took place and proof that they paid on my behalf to Social Security. DO I have ANY recourse at all and isn't there a statue of limitations? I should mention, my employer was California Department of Corrections and Rehabilitation. This dept needs a class action against their personnel dept.