With the Rear Limited Slip Diff, you also need to ensure that the rear tyres are similar in type, pressure and tread depth, to reduce the work the LSD has to do.
Only rear has LSD and 3 litres (2.8 litres some have less) of LSD Oil.
I had a lot of vibration/ noise underneath, it can be the exhaust clamp that comes off the gearbox, you have to fit a double locking nut on this "U" clamp. as it works loose over a short time.
Also if you have a loose under cover/ protector, this sounds like a drum beating, if only one bolt is loose.
But change your LSD... worth doing.
Check the play in the UJ's, grease them, 2 nipples on rear shaft, 3 on front shaft when you get round to it.
Check for fluid leakage out of the rear drums, it could be brake fluid or even gear oil escaping from the rear diff due to a wheel bearing failure ( Happened to me).
Check the play in the bearings.
Also there are lots of bushes and drop links that are part of the rear suspension.
Try rocking the car side to side and listen for clunks, but also check all of the bushes for wear.
Most of the above components show no signs of wear/ movement when they are perfect, so any movement means it is worn or needs adjustment.
Hope this helps.
These are really tough reliable cars, that's why I have kept mine for 16+ years. :thumbs