Here's an odd one......
 

I was sitting in the rig tonight with the engine warmed up. I revved the engine up around the 4K mark and noticed the speedometer would go up to 5 mph, all while sitting in park. Anyone ever seen this before?

Just a guess - check ALL grounds - sounds like you have a back feed

How many grounds are we looking at on a 2000 LT?

That IS weird lol!!! Kinda freaky to be in park and rev up the motor and expect to see the rpm gauge move, but not the speedometer lol.....

Since a couple of months I see the very same thing in my 94 Blazer.

I'm waiting for warm weather to pursue the cause. I'm hoping someone has solved the problem in the past and sees this post.

Got another creepy story for Ya :D
Yesterday I've started moving on from my house. I've been rolling slowly down the private road, when, suddenly, the amber ABS control came on together with brake one. A second after dash went dark, and I'd swear the engine misfired. All just for a half of a second maybe. Then it all came back on, except the radio switch over from a jukebox to a FM station with zeroed clock and the "security" control came on.

That is weird.