can't seem to get rid of dreaded driveline vibration on my 89 300se 41mph to 55mph,is it possible to install driveshaft in reverse order as the half shafts look the same length,the rear half has the u-joint onit as it is right now,i've tried everything,reset center support and tightened slip joint with loaded suspension,moved center support around,vibration speed changes when i move the center support to various positions,it does seem to dissapate as the vehicle warmsup,smooth above 55mph to 70 mph,time to take it to a indy repair shop i guess,must be drive shaft out of balance or bad u-joint,everything is new but no dice,thanks for all the help i received so far.

It sounds like you have it installed correctly. Does your driveshaft have the two bumps for aligning the front half with the rear half?

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Prost!

I would doubt the shaft would bolt in backwards, though never had the urge to try it. The u-joint goes on the rear half, so you have it right.
If you have new flex disks, a good center bearing/ support, and have the bumps aligned on the two shaft halves; then it might be the u-joint.
How does the joint feel? Does it have the slight "detent" when moving off center? Mine used to have the detent at about 100k,, but I checked it last fall, with 200k, and the "detent" was gone, just very smooth through it's movement (probably getting sloppy, just not enough to feel by hand).

Do you have the front flex disc on right - German wording facing the driveshaft.

Are the front and reat alignment bushings on the shaft ends in good shape? They usually last a very long time.

I've been fighting a vibe that occurs under accel at 18 MPH, only when hot.
I went thru all the driveshaft stuff except the u-joint, and never had any effect on the vibe - but yours sounds like a different animal.

Let us know any progress.

DG