Sag on one side is probably not the accumulator -- bad accumulators give you progressively harder ride until they are completely flat, at which point only the tires have any "spring" to them. Unmistakeable.
Sag us either a bad spring or bad strut, and usually one bad strut causes the entire rear end to droop, I believe (I may be wrong).
These struts are not as expensive as the one's in the W123 chassis, but are somewhat more expensive than a shock.
Check the fluid level in the suspension reservoir, too, it could just be low.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!