Hello flexriprock,
If your transmission has never been serviced, the lack of service is what is responsible for this.
Your transmission has to have its oil changed just like your motor has its oil changed.
The interval for your transmission is just not as often as your motor.
Your transmission should have its oil changed every 12,0000 - 15,000 miles.
Don't pay attention to what the owners manual says.
Before you submit your transmission for an overhaul, you may want to try the Range Selector Sensor,
which is on top of the transmission.
This sensor tells the computer ehich gear you are in, and allows the computer to shift the transmission.
If it is not telling the computer when it is in Drive,
it may be the cause of you not having a DRIVE engagement.
If this doesn't work, then it could be the tranny is worn out, or the shift solenoid is dead.