So rather than fix your problem (which is most likely a simple one) you are going to just tear the drivetrain out?

The 4x4 not working is probably due to the shift motor. You can take them apart and clean them up and usually that fixes it. The slipping is probably due to low fluid like Bear River said and the lack of overdrive is due to a solenoid that can either be cleaned or replaced.

To each his own I guess...