Could be a few things....however...the first thing that you want to check is the bushings in the shifter itself. I just had mine replaced and now the shifter is steady as a rock...not only while under torque from the engine and trans...but while shifting...
Feels like brand new....not an expensive fix...just a pain in the butt to get to the bushings. I chose to have my mechanic do it rather than DIY.

Later