2000 chevy impala LS, between 40-60 tachometer bounces hard and shakes the car

you will have to replace tac and speedo should take it the garge

you have a bearing defective !!
it need to be replaced to avoid damage other parts $$$

The 2000 Chevrolet Impala is a sedan available in a pair of different versions, the base model and the LS. The Impala's torque specifications depend on the model, as the base and LS have different engines

Read more : http://www.ehow.com/list_7412083_2000-impala-torque-specifications.html

Limited Series
http://www.acronymfinder.com/LS.html

I think it is located near the bottom of the oil pan on the back side, you will probably have to drain the oil but I'm not sure. That is where mine is located however mine is a 2000 Chevy Impala LS.

You need to test for spark and fuel.
If the coil pack went out, you won't get spark. If the fuel filter is clogged you won't get fuel. Try shaking (bouncing) the rear of the car. Maybe debris is blocking the fuel pickup.

Probably wheel barrow problem, take car to mechanic as it may be dangerous. Thumb me up if I helped. :)

When you changed your tires were the wheels inspected to make sure they weren't bent. If on of your wheels is bent even just a little it could cause your shake. You could try rotating the front ones to the back and see if it makes a difference. I would inspect your wheel bearings and front CV axles and joints. If one of those it bad it will cause your shake.

For the 2000 Chevrolet Impala 3.8L V6:

Firing Order:
1-6-5-4-3-2

Cylinders:
.....(back).....
|...2...4...6...|
___________
|...1...3...5...|
.....(front)

Ignition Coil Connections:
(back)
o <---6
___
o <---3
o <---2
___
o <---5
o <---4
___
o <---1
(front)

Let me know if this helped, or if you have additional information or questions. Feel free to contact me at FixYa.com!

It sounds like you might need to replace your motor mounts, or get a tier allignement done. The allignment you cannot do yourself but the motor mounts you can this requires you to place a jack under you engine and undoing the motor mounts then replace them and take away jack.