I did this job on my 84 300SD back in December....

And having recently done this job, I don't see a way you can SAFELY replace the trailing arm bushings without removing the springs and the trailing arms from the car.

You CAN use the spring compressor that AutoZone rents to compress the springs. I wouldn't use it on front springs (they are mounted in an arc) but it works fine on the rears.

One the trailing arm is out of the car, I created a special tool to press the old bushings out and press the new ones in. Go to a hardware store and get a 2' or 3' section of 1/2" or 5/8" (I think I used 5/8") threaded rod (take one of the new bushings to the store to make sure you get a rod that will fit in the bushing's hole). Also get several large washers and several nuts to fit the rod. You can then (by putting the nuts in the washers in the right place) get the bushings to screw/pop out and and get the new ones in. It's complicated for me to write down, but think about it for a few minutes and I'm sure you can figure out what I am talking about.

I used a similar approach to press in/out my subframe bushings.
Total cost of the tool is less than US$5. It will probably take you an hour to do one side and then 20 minutes to do the other.

Chris Blanchard
1984 300SD 151k