My 2005 4.2 L transmission slips in overdrive but I turn off overdrive and it stops slipping instantly. What could be the reason for this does this require an entire trans rebuild or is it an easy fix.

 

As weird as it may sound, I would check your cop's (coil over plug). If you have a bad coil/s it can often make the transmission slip feeling. I believe the overdrive off changes the firing of the plugs and that is why you may be feeling the difference. I have heard several other people, including myself, have the same or similar problem.