4.10s FTW. I usually travel around 70mph and my rpms are below 2500 rpm. I've noticed that my dad's sport track and my wife's taurus both run at 2400 rpm when going 70mph, so my mustang is right in the average.
Another thing that can cause drivetrain vibration is if the d/s hangs lower on the differential or the transmission. it needs to be even. you can measure from teh ground to the tail of tranny and ground to u joint and try to make them as even as possible, a spacer in the tranny mount can help. You get the idea.
