I went to see a mechanic. He told me that if I replaced the front shock absorbers on my 560 SEL, I would have to replace the rear ones as well. Is it necessary to replace the rear ones? Are the front and rear shock absorbers on the 1991 560 SEL hydropneumatic?

The fronts are normal shocks and are not expensive The rears are hydropneumatic and expensive but rarely need replacing (unless they are leaking). If the rear suspension is bouncy then you probably need new accumulators. Search the topic on this forum for more information.