My 1990 s10 vibrates when in park and giving gas .. And also. When driving but stoped on idle or letting go on the gas pedal it doesn't vibrate .. I have replaced flywheel and torque converter and still the same .. But I also notice that when I pass 40 mph it stops vibrating when it shifts to 40 and starts vibrating again after passing 55-60 mph .. Can anyone help thx

SOURCE: front end vibration

It sounds as though your car has two seperate problems. The first problem with the vibration that goes away when you let off the gas pedal is caused by a bad INNER CV joint. It only vibrates when you are accelerating because that is when the joint is under stress. Your vehicle has two axles, each with an inner and an outer joint. My suggestion is to have a competent shop determine which is the bad side and replace the entire axle. The second vibration which occurs at any speed is most likely cause by either a bad tire, or a bent rim.

SOURCE: vibration at 45 mph.

first, if the milage is high it could be a cv halfshaft even if the boots are not split. if the tires are new and no rims are bent, make sure there is no fix -a- flat in the tires, (liquid will not balance) and cause a severe vibration. i would suggest you take it back to your shop and have them put it on a lift, raise up the front wheels under the outer part of the lower control arm where the front is not hanging down too far and start the vehicle. run it up to 45mph to 60mph and have them feel in the drivetrain for vibrations and shakes.if it is felt with the car this way the vibration is in the drivtrain. inner cv's or in the transmission. thanks and let me know what you find out.

SOURCE: front end vibration

To build upon what emissionwiz posted, the inner boot on the drivers side axle on the Grand Prix's seems susceptible to tearing. If this is the case you will see a greenish looking grease slug everywhere, even up on the bottom side of the hood.

I will second also that the CV joint can cause the vibration you speak of having personally experienced it in a Grand Prix.

SOURCE: vibration in front end

have your tires checked or balanced. it may be time for new tires

SOURCE: My jeep rattle's/vibrates and it sounds like a truck when you let of the gas.

check motor mounts defective ones can cause engine to vibrate