Faulty second hand diff and still having problems

Last year 11/11/2014 I was in a car accident where my Toyota Hilux D4D double 2x4 was struck on the rear diff as you could imagine the diff got damaged in the accident and the diff needed to be replaced. After the installation was done the vehicle has started giving problems first it was the clutch which I had a huge fight with Santam which they replaced after back and forth. Then the second hand diff that they had the panel beater install started giving problems. I am not blaming the panel beater for the problems with the diff as he is only instructed to install the diff by my insurance. Since then the vehicle has been back to him 3 times once for the bearings on the diff which he has replaced and 2 times for seals on the diff. As a paying customer why is second hand parts being installed on vehicles and the parts are not checked for any faults. We pay huge amounts for insurance every month then we get bad service and spares form Assessors who are not fit for the job and who does not even know there is bearings on a diff and they trust these people to approve work being done on our vehicles. Santam service is very bad and they should start checking staff qualifications.