Dodge Durango

I own a 2001 Dodge Durango with 98,000 miles on it in which the motor failed. I got a replacement from a salvage yard with 48,000 miles and had it installed but we are having a very hard time getting it to run smoothly. The motor is a 4.7 and we keep getting a cam shaft code like the timing is off. My mechanic says that if the timing is off that he would have to tear the motor down and start over to reset the timing. Is this true?