I can help you on both accounts, I have replaced both the rear nitro cells and front shocks on my TD. For me most of the bouncy ride was caused by the bad shocks. When I removed the shocks, I found that they were the ORIGINAL shocks from 1983. However they were completely shot. I was able to push the rod in by hand, without much effort. First do not skimp on shocks, these cars are very heavy and will require a serious shock. Stick will Bilsteins, Mercedes has for the last 40+ years. This is what I did. With the car on the ground, loosen the nuts on top of the shock. Now, jack the car up, remove the wheel, and start removing the 2 bolts holding the shock to the spindle. These are 12 point, 10mm bolts. Installation is in reverse order. Do this first and see if this illiminates or reduces your bouncy ride. The rear nitro cells are more expensive than the shocks. Does the rear of the car sag at all after it's been sitting for a while? The design of the car makes the backend look like it's sagging so to be sure the backend is at the correct height,do the following. Take a ruler and measure the distance from the jackpoint hole to the ground at all four corners. Of course make sure the cargo area doesn't have anything in it. They all should be about the same, give or take a little. If the back end is indeed sagging, check to see if the rear hydro-pneumatic shocks aren't leaking ($$$$$). If they are dry, then your nitro cells are bad. To replace these, you will of course need 2 new cells, one for each side, and replacement hydraulic fluid. You will also need a 11mm flarenut wrench. Do the front shocks first, and if that doesn't fix your problem, post what you find on the backend and I'll tell you how to replace the rear nitro cells.
I replaced mine due to the sagging.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member