2005 Hummer H2 Computer reading shifter

2005 Hummer H2 Two Wheel Drive Automatic 42,000 miles

Before I got my Hummer it was put on a rollback because they didn't have the key. I think something was taken loose on the shifter. When you go to put it into park you have to do it slowly so the computer can read it. Sometimes you have to try two or three times. If it doesn't go into park and you turn the key off it won't crank the next time. How can I fix this.