So I got a new truck on Sunday. I've noticed that after driving it more there is a pretty persistent vibration that seems to occur when I'm at around 1400-1800rpm in 3rd and 4th gear. It doesn't occur if I am simply coasting but it gets worse if I push on the gas until it reaches around 2000rpm. I havent noticed it in any other gears but it is pretty loud. There are a few modifications. The previous owner had a toyota front diff put on as well as a completely custom exhaust system. Big tires too (not exactly sure what size). Could this cause the vibration? I'll try later to see if it does the same at the RPM's in neutral.
Get under and check all of your mounts, joints etc.
Jack up either diff and give her a spin around by hand.
Could be something as simple as out of balance wheel or alignment, does your steering wheel wobble or anything?