I'm pretty sure it is your drive shaft u-joint. They get a little notch worn in them that's difficult to see/feel by visual inspection. Been thru this with my 436k mile SDL.

You can test by putting a couple washers between one of center carrier bearing housing flange bolts and the car mount point to change the angle of the drive shaft at the u-joint. If the vibration changes or goes away you've found your issue. Have the u-joint replaced & shaft balanced. Ran me about $300.

Otherwise the list is long...OE motor mounts, trans mount, center carrier bearing & housing, flex disks, differential mounts, & of course axle shafts(pull & reverse R to L).

Since you've ruled out wheel stuff, and with your lower miles I think it is just the drive shaft (seat vibration) but all the power train rubber is probably going south.

P.S. buy good rubber like Lemforder, Phoenix, or dealer...knock offs like euro don't last.

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1986 300SDL 440,xxx