The manifold on my passenger side is cracked all the way around and when i start the truck it idles rough and gets rougher when i put it in gear it gets worse. It evens out when i give it some gas and then its fine.

 

in short, yes it will. it will give the comp a false o2 sensor reading causeing either a lean or rich mixture causeing the computer to compensate for it,

 

But wouldn't that cause a check engine light.

 

Not if the O2 was within operating range as viewed by the PCM. If there was a huge difference between banks (adaptives) then it would throw codes. I'm inclined to think the problem is a worn motor or trans mount in this case.