Hey guys, i have an 03 G35 coupe with 171k miles. The issue here is that i'm experiencing a slight vibration underneath the pedal as soon as i hit 60 to 75 mph. The sound of it is not extremely loud and it makes a thud... thud.. thud.. sound. Could it be that the driveshaft is going bad?

 

Could be a lot of things.
Either take it to a local shop to diagnose, or do a process of elimination starting with better defining the conditions that the vibration presents.
Since you say vibration is felt in gas pedal and this isn't a X model = rear end.

If you can safely get the car up to speed and then coast in neutral for a short time - the vibration will stop if its input related: trans/shaft/diff/axles.
If the noise/vibration continues with no drive input would suspect out of balance or not round tires, bent wheel, poor alignment, or wheel bearing.

 

By "...underneath the pedal...", do you mean the gas pedal, from the firewall or floor board? Does the noise come from any other locations, i.e underneath the brake or clutch pedals, near the trans tunnel? Does your area/city have any quality chassis shops?

As already stated, you should have it checked out professionally. Be safe!

 

Found out the culprit to the issue.. The ball joints are completely shot and will be replaced asap!