driveline vibration

[Rob czep]

Anyway, I have this bad vibration at about 3000rpm when I let off the gas, its fine when im on the gas (under load). Im sure its coming from either the driveshaft or one of the rear cv joints but theres know visable damage to the boots on the cv's. I replaced two u joints on the driveshaft and the third is fine to see if that was the problem. I made sure the driveshaft went together the same way i took it off. So im thinking its got to be a steady bearing or that weird joint in the middle of the driveshaft (not to sure what it is called). I was wondering if anyone has experienced this same problem.  thanks!
                                            Rob