Bent transmission output shaft?

[Zhanshi]

I have a 93 240sx, it was originally auto, but was swapped to manual last week. As soon as everything was installed I started the car. After shifting to first gear, I noticed that the shifter would start to rotate in a circle (counter clockwise). This only happens at low rpm and starts to diminish at higher rpm. Is it possible that the output shaft is bent? Since at higher rpms it reaches a sort of "ressonance frequency" so the rotating shifter isn't as violent? The gears 2 through 5 and reverse shift smoothly, but at times, first gear requires some force to engage. Thank you in advance!