It won't go into 4th Gear

Hi, i have 03 Aztek AWD. It has about 170K miles and had an accident in the rear quarter driver side. So I about the car from a guy and fixed evrything and all that, but now the car won't go into forth gear. It shifts fine and doesn't seem to have any problems, but it just wont go into 4th and there is no light(check engine) on. If anyone knows what the problem is or have come up with the same problem, a sensor or solenoid which i think it is, please help me out here. Thanks in advance.