1994 Cherokee shake/vibration @ highway speed

I have a 1994 Cherokee that shakes at highway speed. I had a similar
problem a couple months ago and replacing the front drive shaft
u-joints fixed it. Problem is it's happening again.

I checked the u-joints on the front shaft and they appear fine. Tires
are only a couple months old and have been rotated and balanced
regularly. I just finished replacing the wheel hub u-joints which were
bad and the shaking continues. The shaking occurs in both 2 wheel drive
and 4 wheel drive. Does anyone have any idea what might be causing
this?