The car in question is a '92 240 with approximately 240k miles.

I replaced the driveshaft support bearing on said car last night with a new FAG brand bearing, Meyle support bushing, and new spring. I marked the junction of the DS to rear diff and the halves of the driveshaft with whiteout before I took anything apart. I took this measure even though our car has the factory arrows on the driveshaft to help with alignment.

Everything seemed to go smoothly. The old bearing came off without too much trouble, the new one went on pretty smoothly, and everything lined back up pretty nicely. Car repairs never go so smoothly, so I knew something was amiss.

Sure enough, I went for a test drive and found that the car now has an obnoxious driveline vibration at all speeds. What should I be looking for here?

Could there be something obstructing a full mating of the two shafts at the center support? If so, that could create a pre-load condition on the shafts that is more pronounced when you have weight-on-wheels.

So if you raise the car up and support the rear end by the axle rather than by the chassis (normal "driving" position), you should be able to unscrew the yoke flange bolts several turns and EASILY separate the flanges without finding it difficult to compress the slip joint at the center carrier. The slip joint needs to be able to easily compress when you load the car down with fuel/passengers/groceries/firewood allowing the overall driveline length to expand/contract with the variations in live load.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

What shape were the U joints in? No mention about inspecting them.

It could be old or dry out of grease.

Being the needles are well worn, you may have shifted some of the needles in the one or more of bearings cups. This can easily happen while the shaft was being flopped to get the bearing off.

Now after driving and reloading the flat sides of the needles they have shifted and frozen up the joint.

If this has happen it won't take long for it to get worse and start locking up.

Just a thought as the shaft was the last thing touched.

Phil