Vibration in 2nd between 1-2k RPM

I have noticed very faintly for the last week that my 05 V6 has had a vibration type stumble in it. It only happens in 2nd gear and from about 1k-2k RPM, then shifts out to 3rd where its smooth. It almost feels like the back of the truck is going over some minor rough road, and thats what I thought it was until I detected the pattern.
Its somewhat baffling me since its only in 2nd gear and not any others. I replaced the plugs 3 weeks ago with OEM Denso and cleaned out my K&N. Oil is due soon, but again, its not consistent amongst gears.

On a side note, I noticed this morning that my heater is only pushing out hot air when I am on the gas, when I coast down, or sit idle, I'm getting cold air.

Its probably totally unrelated, but thought I would put it out there.

 

With more driving and testing, I have noticed its more speed related than RPM. Right around 13-17 mph and again when braking down through that speed range.
Maybe tires, driveline?