Overdrive issue?

[Vellers]

At about 40+ mph the rpm drop down to 1500ish and the car starts to vibrate.
If I give it more gas to kick it back up to where the rpms are while I accelerate (2k-3k), its smooth as butter... It only happens to shake when the tach needle settles.

Its a 2002 LS V8.

Hopefully its not a big issue/expensive to fix.

 

[NYC LS8]

Could be a coil or two on the way out, but not dead enough to give you a check engine light. When was the last time they were checked/changed?

 

[Vellers]

I changed the valve head gasket on the drivers side, along with all the coils and plugs (on that side). As well as cyl 1 on the passenger side because it was throwing me a misfire code.

 

[oddball]

You should change the remaining coils and plugs as well, then re-evaluate.