Tumbling/viabrating at 3500-4000rpm

[Birdman12]

Hey ive noticed that when i get up to about 3500-4000 it feels like the driveshaft is about to fall off or something there is this crazy rumble/tumble kind of sound and feeling comming from near the back of the car. It's especially bad when im on the highway because cruising speed is withing 3300-4000rpm for me on my sr20.

so any ideas? a couple friends said it might be the driveshaft mount or something or even a twisted driveshaft? it runs smooth in all rpm ranges except between around 3500-4000rpm. thanks.

[Birdman12]

oh its a 92 hatch. hicas was diconnected and a lock-bar put on.

[Z U L8R]

could be the center bearing on the driveshaft coming apart, u need to get ur car in the air and take a look, i'm sure you'll visibly see what's going on