I was at the bank today and when I put it in reverse to back out, the car jolted and then it rolled forward slightly while at the same time sounding like there was grinding. I immediately put it back into park and the noise stopped. Then I tried to reverse again and it started making a clicking and whining noise and wouldn't back up. It sounded like it went into gear but it won't move. I was able to get it pushed out with the help of some friends and the drive still worked. So I was going down the road and got up to speed and the shifting was significantly delayed. It's an automatic by the way. So I tried to take it on the highway and it won't shift out of 2nd gear or 3rd. I know for a fact that overdrive isn't working so i'm doing 50mph down the road and hitting 2500 RPM. I don't know what the deal is but for about 8 months there has been a rumbling vibration when it tries to go into overdrive and when I accelerate quickly I force it to downshift and the rumbling goes away but it still shifted into overdrive when it got passed the rumbling and vibration. Any ideas on what happened? It's a 1997 C1500 2WD 5.7L Silverado.