OK, look. FORGET bell/ clutch/ pilot alignment FOR NOW. If any of that was screwed up, it would be hard to shift, or hard to get into gear, etc
I think you either have a yoke/ ujoint or a balance problem.
Check carefully the play between the yoke/ bushing.
Check that the yoke is not bottommed
Pull the shaft and TAKE THE U JOINTS apart and inspect, re-grease, and re-install HAVE THE SHAFT BALANCED
Check your pinion for play in the bearings, or loose on the shaft.
Just for drill, try a different set of (borrowed) tires, or at least the rear pair.