I have "new" 92 400SE...When I purchased the car I noticed that it had this vibration at 50-65 mph....Nothing higher...and nothing lower..just in this sweet spot....It seems to be coming from the drive train but I am unable to nail it down..Looking at the parts everything looks ok. Suppose it could be the Engine , tranny or DS mounts...Fr and RR Guibo's look good.How do I check....For now I just drive either faster or slower and stay out of that zone

could be the driveshaft bearing,if it has one.Sound about right for the age
.If you drive thru high water and vibs get worse its the bearing