I've read a ton of posts regarding scrolling credits and how to make them smooth (various combos of blur, flicker reduce, expressions), but I am having no luck.
What I have been given is a psd with the text layers merged to retain the specialized font the title creator used. SO, I took the psd and keyframed it in After Effects to scroll from bottom to top. The problem is that it looks jumpy. The only fix is if I do this in 29.97. Then it's perfectly smooth. When I do it in 24fps, there's a judder to it.
What am I doing wrong? Or is this something that happens in 24fps.
24 fps will do that - it's at the very edge of human perception, where the movement starts to break down and the eye perceives individual frames instead of smooth movement. Add motion blur, if the one that is generated by the movement of the layer in AE does not seem enough, try CC Force Motion Blur. Still at 24fps you will notice judder.
If the speed of the scroll is constant, you may find more pleasing (and faster) results by just adding a vertical directional blur to the text. 24 FPS is overrated... I think the only reason to use it is if you are actually doing a film-out.