Noisy rear suspension - possible causes

[Meridivs]

Recently my rear suspension has started making odd sounds (something like deep thuds). It sounds like when you have something heavy bouncing around in your trunk. The car is a 1984 300SD with @ 220000 miles on the clock.

I've visually inspected the shocks and they don't seem to be leaking or noisy when stationary.

I wonder what would be the cause for those sounds and what I should do to fix it?

Is this car still driveable in this case or should I change the shocks ASAP?