I’ve been driving for a few thousand miles after my rebuild, and in the past week, the car developed some vibration. The car starts and runs fine and revs up without problems, but when driving and the car hits around 25 miles an hour, the steering wheel, dashboard, shifter, and pretty much everything else begins vibrating. The vibrations continue as the car speeds up, but they are less violent because it’s vibrating at a higher frequency. The vibrations are the strongest at 30 miles an hour. If I release my foot from the gas and continue at 30 miles an hour the car stopped shaking Also, if I engage the clutch, it stopped shaking. Or if I accelerate the car significantly, the shaking stops. It shakes when maintaining a constant speed or barely accelerating. It’s not any tire or suspension issues because I put the car on. Jack stands and ran it in gear showing the video below. I checked you joints and driveshaft connections. Everything looks fine no obvious issues or dent in the driveshaft or missing balancing weights. I checked engines and transmission mounts for vibrations and they feel solid. It’s possible that the driveshaft is bent or warped, but it’s not very significant if that’s the case. The video below is me running the car on the jack stands it’s in third and I’m holding it about 25-30 miles an hour. I have a full video, but it’s too long.
Not sure what else it could be. Any advice is welcome. Thank you!

 

Interesting..... I would still check that your lug nuts are tight and your bearings aren't shot. That's what happened to me, not one but both, got lucky catching that.

You do need to orient the drive shaft in a certain way. If you take it off and rotate it one bolt away from the starting point. Do that until you are back to the beginning, you should know for sure. Also, check for slop in the u-joints. I am not sure if the diff would cause something like that or not.

Just on the outside chance that you replaced the flywheel with an aftermarket one, some of those require a counter weight to properly balance the engine.

Going with the shacking stops while the clutch is engaged. Let hope that you put the clutch/flywheel back together correctly.