2010 Tahoe stumbles in automatic drive

My 2010 Tahoe was taken in for repairs because it stumbles whenever I
accelerate then smooths out somewhat once it is going.  The check engine
light is NOT on.  The auto repair place initially said it was the torque
converter, but later on they stated it was not and that they could not figure
out what is wrong.  However they were able to establish that my Tahoe
does not stumble while driving it in 2wd and 4wd.  At this time I'm driving it
in 2wd and the drive is smooth.  So what is causing the problem in
automatic?  Will I eventually encounter problems while driving in 2wd?  
Help