My 1995 Chev 2500 pickup won't shift automatically, I can manually shift between second and third but first an OD don't work. Also, (this may be related or not) the tach and speedometer dont work.

Do you know if you have the 4l60e or 4l80e transmission? The trans uses the vehicle speed sensor to shift. If it doesn't work then the speedo won't either. That could cause this. Also sounds like a bad solenoid IF it's the 4l60e.

My wife's MBZ CLK wouldn't shift. There was a leak in the transmission and she was low on fluid so the transmission went into a survivor mode by refusing the transmission shifting out of third. I had her tap the shift lever to manually shift it up, which bypassed the survivor mode. Check the fluid an see if there is any fluid leaking from under the truck when you move it after parking for some time. It does sound like a sensor if the speedometer and tack also does not work unless it is all going through a computer and it is shutting down.