Engine/Trans Idle Trouble.

[Mgizler]

Ok guys, Im having some trouble with my truck. First off its a 1984 GMC K1500 4x4. With a 350/TH350 in it. All Stock. And it was running fine until the other day I was going up my back yard and hit a large bump, and it knocked it into neutral and started runnign at a very low idle, like it had a huge cam in it.  When I start it up, It idles fine in park, but as soon As I shift it into reverse or drive, it drops drastically. To like 400 rpms. This does not make sense. I checked the timing and its not off. I put new plugs, wires, cap/rotor on and still doing the same thing. I also re adjusted the carb and still no luck. Let me know what you guys think.
Thanks.

[Captkaos]

Check for open vacuum hoses and leaks.