Straightness, either due to a new shaft not being built straight or a used shaft being bent.
You would need a dial indicator to measure such radial run-out and that would probably best be done at a drive line shop, because after straightening, the shaft will likely need to be balanced as well.
Could also be due to a u-joint not being installed properly or a drive shaft flange not fitting properly causing the shaft to not line up with the output shaft or pinion gear.
And realize that drive shafts are typically straightened by use of heat and cold. The drive line shop will use a torch to heat one side of the shaft, expanding the steel on that side, while cooling the opposite side with a wet rag to shrink the steel there to pull a shaft back into straightness. That said, any application of sufficiently high heat can case shaft to lose straightness.
Vibrations at relatively high speeds are often due to balance problems.
These vibrations may come and go as speed increases as you move in and out of resonant frequencies of the drive shaft.
Vibrations that get worse when transitioning from acceleration to coasting to deceleration, like when you back off the gas over the top of a hill and before you go into full on engine braking, may be due to loose or worn parts, like loose flange bolts, worn or over-extended slip yoke, etc.
Vibrations that get worse say going uphill or accelerating at speed than when going the same speed on the flat or downhill, or vice versa, might be due to a slight alignment issue.
If worse uphill/accelerating, the lower u-joint angle may be moving too high as the axle and pinion tip up under load, if so, tip the static pinion angle down a little lower than it is now.
If worse downhill/decelerating, the lower u-joint angle may be moving too low as the axle and pinion tip down from the lessened load, if so, tip the static pinion angle up a little higher than it is now.
Vibrations at very high speeds may be due to approaching the drive shaft critical speed, which is essentially the resonant point where the shaft begins to whip and vibrate violently.
In this case, only a new driveshaft design will help, changing material, tubing diameter or wall thickness, etc.
These sorts of RPM limits are usually up in the 8000-10,000 RPM range, so typically only seen in race vehicles.
