Had a co-worker move the steering wheel back and forth today. I'm pretty sure it's the lower U-joint.

I had this same problem a few years ago. Turned out that front wheels/tires needed to be balanced. If I recall one of those little balancing weights had come off. Completely solved problem by getting both front tires balanced. Cheap repair. Think maybe $15 total.
--
' 91 740 sedan nearly 157,000 and ' 87 240DL auto nearly 135,000

You didnt mention if you got an alignment recently. Being off toe can give you that shudder also.

I would inspect the ball joints, tie rod ends, etc. closely anyway. Just because they're relatively new does NOT mean that they are still good, especially if they are aftermarket.

Check the lugnuts for signs of looseness. Swap your tires back to front one at a time to see if you can isolate it to a bad tire or unbalanced rim.

Confirm that your steering rack is still securely fastened to the crossmember with those u-bolts.

john

No more than you've said, I would start at wheel balance ( threw a weight?) and go from there. If it is, it will only be the front ones as the passenger seat (unoccupied) would shudder too if it were rear wheels. Second, I would look at the tires, as they alone can cause wobble if they are defective. Good luck.