Besides the above, there is also the possibility that with all the vibration it is subject to, the transmitter has gotten out of adjustment. That is one of the biggest reason truckers have to replace their CB's frequently. If that is the case you will need to take it to a CB specialty shop. When you test your SWR and you can't get it below 1.6 even with a different antenna then that may be the case.
MK2 CB's show up on ebay all the time, and I've even purchased one for $30.