Runs for a while than shuts off if i take the back off it doesn't shut off.

Usualy this is caused by poor airflow through your dryer. Remove the lint filter and check for a lint build-up near the bottom of your lint filter. If you see a lint build up use a vacuum crevice tool and something long like a fly swatter to loosen the lint as you vacuum. Also check the dryer vent tubing and the outside vent flapper for obstructions. What is happening is your dryer is heating up until the high temp limit protection rhermostat shuts off the heating element.

What I just mentioned is apparently not your problem. You said that your dryer runs then shuts off completely. Check to see if the motor is easily roated by hand with the belt installed. If it is difficult to rotate there is a bearing problem somewhere causing heat to build up in your motor until the thermal overload shuts off the motor. There is a drive belt idler near the motor that can also cause excessive drag on the belt if the bearing has failed. The last thing to check would be the drum rollers located in the groved area on the outside of the rear of your dryer drum. If these have stopped turning they act like a drum brake and can also cause the same symptoms. Removing the back panel allows the chassis to shift enough for the drum to ride lighter on the drum roller even though it may not be turing it is enough for the dryer drum to skid / slide on top of the stuck drum roller yet not shut off.. Hope this helps and good luck. I can help with access issues need be here.

One question I have is does your dryer door open and close properly? Some times a bad door seal or door catch can cause the same symptoms. Open and close the door and losten for a switch click as the door opens or closes not far from the closed position.