You could check by yourself if the shock absorbers are worn - press the rear end of the vehicle towards the ground - if it comes back up and rests there without or only very less shaking up and down, the absorbers seems to be quite well. If it shakes up and down few times after pressing it down the absorbers are worn. To be sure 100% let the absorbers check by a shop - because the self test is no guarantee for 100% correct shocks it only indicates that the shocks are ok. I had the same problem and replaced the coil springs, the shock absorbers and later on the complete rear axle bushings which could be the other cause for a sagging rear end.

Failure of the rear bushings is quite common on the 240s with high mileage which causes the sagging rear end syndrome. If they are worn you´re going to hear a clunk sound from the back of the car mostly everytime when you start driving after braking down the car or drive at extreme low speed. Moreover the car will maintain this sagging rear look also without any load or passengers inside it.
I think there are 10 of these bushings. There are the track bar or panhard rod bushings (2), the reaction rod bushings (2 on each side of the car) and the trailing arm bushings (2 on each side of the car).

The kind of sagging you have described sounds like your shocks and/or the coil springs are worn. This job you could do quite well on your own without any special skills - the only thing that is necessary is a spring compressor to lift the spring out of its position and the correct size nuts to remove the bolts of the shock absorbers.
The rear axle bushings you could not change by yourself because a special tool is needed to press the old ones out an the new ones inside the rods and trailing arms. Take a look at www.ipdusa.com there you will find shocks, coils and the rear end bushings with the prices. I would recommend to check and if necessary replace the shocks and the coil springs before you going to work on the rear end bushings.

Good luck -- have a nice day

greetings M.P.