I've been chasing this since I bought my car but it recently got worse. I'm trying to decide what the next replacement part will be and maybe see if anyone else has gone down a similar road.

Parts: z1 poly engine mounts, poly filled trans mount, white line diff bushings

OE 130k mile everything else

Symptoms: I get vibrations at 2000 RPM ever since I installed the poly engine mounts. Recently it started vibing hard on engine braking from 1.5 to 4k. There was also a feeling of driveline looseness in all gears similar to bad engine mounts. My shifter was moving around despite the new rubber mount so I put in a poly filled trans mount.

After, if felt as tight as my buddy's e46 except for the 2k vibes for about 2 hours, but it seems something else has gone out to compensate. The vibing on decel and loose feeling has returned except this time with no shifter movement. No scary clunking noises or anything like that. Like before, driving around at low gear at high RPM my revs jump a little coming on/off throttle...

No movement with the engine, u joints are tight, center bearing isn't visibly damaged.... Any thoughts on how I should proceed? There are two worrying things I noticed under the car: There is a tiny bit of movement in the connection between the transmission and the driveshaft. There's also some movement in both of the bearings where the axle shafts connect to my differential. Could this be causing the problem?