I have a vibration coming from the rear end. Its related to the differential.
I know this because last week I changed the diff fluid and the vibration went away completely. But yesterday I took a corner (prob marked as 30mph or slower) at around 50mph or so...the vibration returned (although not as violent as it had been).

The vibration occurs once the car has been driven and warmed up a bit (i noticed once that when driving in the rain the vibration was nearly gone).
It kicks in at around 50 mph and gets progressively worse as my speed increases.
however, after the vibration appears, if I slow down below 50mph the vibration will remain present. After it gets bad I can even feel it in my shifter.
(now that the vibration has become very violent I can feel it at all speeds. Even after sitting for a few hours and driving at 20 mph, i can still feel a slight vibration)

I have checked my rotors (light braking while in motion) and they are not warped. Further, after I changed the fluid (RP GL5) I could brake with no "warped rotor feeling"
I only mention this because once the vibration has started it gets worse and worse the longer I drive it.
After a certain amount of time I can start to feel it while breaking.It keeps the same "vibration frequency" while braking as it has wile driving (if you were to only feel it while braking you would think I had warped rotors).

I pulled the PPU and checked the Driveshaft and U joints. They all felt tight.
I checked the axles and they both felt tight.

My friend suggested it was a "prop shaft," but I have no idea what that is.

I was thinking, some kind of bearing....? maybe a differential output shaft bearing thats connected to the driveshaft....
I would also like to reiterate that the vibration went away COMPLETELY once i changed the fluid and only returned after stressing the diff while cornering.

ANY useful advice is greatly appreciated.,
This is my first car with a rear diff and I'm really at a loss>
I firmly believe that changing out the whole thing would fix the problem, but I'd much rather isolate the actual problem and address it personally.

Thanks again,
Matt

 

prop shaft, propeller shaft and drive shaft are all the same thing.

so the power plant frame, (you say PPU?!), is adjustable, so it actually does affect the phasing of the driveshaft, which could cause a vibration if its off.

there is a spec in the FSM, you put a bolt in the corssmember and set the height of the transmission, Foxed.ca - Mazda RX-7 Manuals

that being said if the symptom changed when you changed the oil, that doesn't look good for the differential assembly

 

Did you check the old oil you drained for metal particle's , if you have access to a lift/hoist, throw it on and run the car in gear while someone checks underneath for the vibration. Be careful of loose clothing and long hair when checking.

 

thanks for responding guys.
I'll check the specs on the PPU ( I call it PPU - power plant unit - because that's the first name I had ever seen)
BUT I had the PPU on and off before I changed the oil and the vibration never changed.
It was only after I changed the fluid.

There was A **** TON of metal...thats obviously an opinion...there was about a marble sized "ball" of metal I pulled off the magnet of the drain plug and it seemed as though there was a substantive amount of metal shavings in the drained fluid...but this really feels like a bearing of some sort.
My rationale is that the problem is exacerbated after the car warms up.
(i think, the grease in a bearing, when cold, is more viscous and will adhere to the bearing itself better (similar to why some use higher weight oil in the summer). THen after it warms up a bit the grease becomes more fluid and the bearing starts to heat up and cause a bad vibration...idk something like that....I'm really at a loss, but since it doesn't happen ALL the time...its hard to be specific).

 

I have a lift, but its a four post and I can't hang the wheels...

 

I looked at the FSM and if i were a gambling man (oh wait...I am...come one Hard 4!! lol)...i'd say it was the front bearing (#22)

Has anyone had a problem with this part?