Car rear end bounce even after changing the shocks

Sometimes Rear end bounce when I go over 60 on highways, this happens mainly when I hit uneven parts of the road, and it happens on highway bridges and concrete roads.

So I changed the rear shocks, now I see the amount of bouncing has reduced but it still bounce more often than before.
Mechanic said the coils are rusting, do yo think that could be the reason? do I need to change the coils. please advise.

Car rear end bounce even after changing the shocks