sloppy differential?

Starting at about 30mph, there is a hissing noise from the rear of my '71 250C. It is load sensitive (i.e. present at accel. and decel, but temporarily gone between the two). Also, when I turn off the ignition in park and release the brake, the car will roll back about 6" on an incline. After I searched the archives here, I replaced the drive shaft carrier bearing and both flexible plates, but the sound is still there (the ride is much improved, however!). My next guess is the differential or CV joints. Just replaced the diff. oil, but how would I test it to see if it's the culprit? Could it be something as simple as rear wheel bearings?

[This message has been edited by wjbell (edited 02-07-2000).]