I have a 07 Chevy tahoe..the rear door opens halfway than closes??

{: ) Try to disable the power lift system , there is an on/off switch located at the center console or overhead console. Then try to open it manually and see if it will hold at full open. If it falls back, then you have a weak liftgate strut, and you will need to replace both.
Or if you can't disable it for some reason, you can give it extra help while on its powerlift up but dont force too much. If it goes up successfully when you applied little help, remove your hand and check if it goes down. But if you feel that the liftgate wants to return back while halfway, remove your hand and let it be.
Was there a time when your liftgate is open, then starts to close and then reopens again at 1/3 on the way? The cause of this may be a sharp sheet metal edge inside the inner roof panel chafing the liftgate harness. And if you got B153 codes just in case, you may do some wiring checks and remove the panels at the rear roof area and look for that gray connector with 22 pins located under the center of the rear headliner panel.