Miata Turbo Forum - Boost cars, acquire cats. - Is this noise and vibration coming from rear axle?

Need some input/ideas as to what my issue might be. I've got a 90 miata, had my baby for 10 years. Recently swapped out rear ends from a 2nd 90 miata that was wrecked with front end damage and even though I told my dude to put the original rear end back in my car,,,well I'm not sure he really did. I think he thought he could fix the issues. My rear end did not have the issues the other did. Anyway, here's what is and has been going on.

City driving, can't tell there's any problems. Drives great, no noises, no vibrations.

Highway driving. Starts up slightly around 65 but stops at 70 only to start back up at 75 with noise and vibration far worse and gets worse the faster the speed. The noise is an extremely low pitch, so low it makes my ears feel like I've jumped up suddenly in elevation. At 75 mph is when it starts effecting how my ears feel. The vibration reminds me of when a car drifts onto the right shoulder and runs on the ruts in the pavement, just not as loud and harsh but very simular in feel and sound. When I look into rear view mirror at night the head lights behind me are vibrating left to right and the lights trail in this fashion so as to look like a solid longated light. The vibration can be felt coming from seat and you can almost let your teeth vibrate along with it. Sorry guys but no it's not the kinda vibration that puts a smile on a woman's face. It's more of a panic look,,,LOL.

Any input would be greatly appreciated and before anyone says wheel bearings, already checked for movent in rear wheels and they are solid.