I am getting ready to replace the shocks on my 90 420sel. I really at this time only need the rear replaced, but have considered all shocks. I am going to replace with Bilsteins. My question is should I replace all four or just the rear? Also, should I go with the regular shocks or heavy duty?

...will no doubt make a real difference if your old shocks are worn out.

Unless you are towing something heavy the regular shocks s/b fine for another decade, or so...

Actually the US made shocks that are available, for example, the Monroe LE10134 @ $85 per rear set, are (limited) lifetime shocks @ half the price.

With a good installation & a 4-wheel alignment you should be in business

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435

Hi Scott,

assuming that the front of the car went over the same bumps as the rear, the front shocks have suffered the same amount as the rears
If they have signs of oil leaking, I would replace them.
I changed all 4 on my 87 and I used the regular Bielsteins. The car is mostly driven with one or two people in it and I prefer a cushy ride.

When I took the rear back rest out to reach the top shock mounts, I also put some carpet strips on the metal back and the floor. It dampened the road noise quite a bit.

Good luck

Reinhard