Consumer Portfolio Services CPS Truck is paid off 60 payments . CPS purchased my loan before payoff and now say I still owe 2600.00 on it. Irvine, California

We purchased our truck in December of 2006 on a 60 month agreement.  Our loan was sold to Consumer Portfolio Services in September of 2011.  Our Truck was scheduled for loan Maturity in Jan 2012.  We receive a notice saying the Loan is set to mature in May 2012.  We paid ahead on the Loan on several different occasions and stated on the check the months to pay for.  Neither Fireside Bank nor CPS applied payments according to the dates of pay on the checks.   Upon purchase of the Loan CPS sent out the amount still owing in September showing 4600.00 on the truck.  The letter said that if we disputed the amount to contact them of which we did.    CPS called tonight stating that the truck was up for Recovery and that if I paid 810.14 on the truck that they would continue the dispute on it for what they called Interest.  They also say that they have a different contract on file than the one we have on file stating the purchase in December of 2006.  Their's states May of 2007.  Still the truck would have been paid off by now.  I filed a complaint with the Division of Finance through the State of Florida.  They have other complaints concerning the same thing.  They are stating in one breath that we have made 58 payments on the truck but yet they state we owe a little over 6 payments in dollar figure.  We have paid over $24,600.00 on it.    We have ordered copies of our payments from Western Union, and the other 2 banks because of our Moves in the last 5 years.  They state that they don't care and are planning on trying to pick it up this weekend or turning it over this weekend.  It has over 400,000 miles on it and quit running last week.  It is not worth any more money and I don't want to pay any more.  But to save our Credit I don't know what to do.  Please help.