recently, when the truck kicks into overdrive, and the engine drops below 1500 rpms, and I step on the gas, it makes this sound that the only way i can
describe it is like a low growling sound. the sound goes away at around 2000 rpms. anyone have any ideas what this might be and if its something that should concern me?

 

Mine does the same thing. Regular cab, 4 X 4, SWB. I can hear the sound a lot better with the back glass open. With mine, the growling noise starts when the torque converter locks in overdrive (45 mph) and goes away around 55 mph or higher. I started noticing it around 100k miles, have 180k now. The sound has gotten a little louder over time, but still isn't terribly noticeable with the back glass closed. I initially thought it was a trans issue, but the sound persisted after the trans was rebuilt. I have decided the noise is in the rear axle. Maybe the pinion bearing is worn, has a little slack in it, and only makes the noise under that certain load level when OD lockup is first engaged. At any rate, if your issue is the same as mine, it has caused no issues other than an annoying sound for 80k miles. I do have another 3.55 ratio rear axle squirreled away in the event the sound suddenly gets more serious or if I get tired enough of hearing it to do the swap.