petes right if u have poly motor mounts or trans mount the vibrations are bad. i put a poly trans mount in mine and i though i did something to the motor or drivetrain. changed to a stock one and is fine.

 

not sure on the mounts they are just oreilly 9 dollor mounts name brand anchor

 

i got out on the highway and in third gear at 55 it shakes really bad weather you accelerate or decelerate except for under hard eceleration it smooths out, kill the motor or take it out of gear vibration goes away. definitely coming from somthing in motor more like burnt valve or somthing cam lobe wore down somthing along the lines of that compression check turn out good and even

 

sounds like it could be the carrier bearing.

 

idk but the more i drive it and try to get it to vibrate to see what it is the more it keeps going away and sounding better, that could either be good or bad lol
im starting to think its a busted bell housing or the tranny bolts are loose, or the egr tube to the exhuast manifold is got a blown gasket it seems like it keeps coming loose and teeters back and forth maybe a new gasket would fix that. I hate to use locktite on the bolts thought on the exhuast i dont now if thats a good idea or not maybe ill get lucky and that will be it. it seems like the simplist things are the hardest to find somtimes

 

if u have a two piece driveline get under the car and see if the driveline will wiggle in the middle where there is a support bearing thats called a carrier bearing, ther are also extra ujoints there. i have had a carrier bearing going do just what u are saying. vibrates at a certain rpm with a certain amount of pressure on it.

 

it does wiggle but only the rubber is moving bearing feels good were did it sound like the sound was coming? im hearing it towards the front passenger side.

 

sounds like its good. fyi u can use a 1 piece driveline if u want to. its a cheap upgrade. if the cars running good i dont think its in the motor. when it does this is the motor running on all cylinders and sounding goood? something out of balance would do it all the time in gear or out. being its only doing it while in gear and under certain conditions i would say its in the drive system. it could be somthing thats allowed to make contact with the body. have u tried holding the brake and giving it some gas like u were power braking but without letting the tires spin. this would be to see if u can get the motor or trans to move into the position to vibrate. im just shootin it the dark here so bear with me. lol. if u can get it to vibrate this way u can get someone to look for it while its happening.

 

no it dont missfire or anythang seems to run good. I tried the power brake and it never vibrated or either that if the motor is hitting body it couldve tourqued on it hard enough to not give it a chance. motor sounded good no cutting out or anythang under load, and thats just the thang i wish i could explain it better. i checked for anythang hitting on body from top and didnt see anythang

 

sorry man im stumped. is it a auto trans?