I thought I had the same problem with my '95 850T wagon as all I could hear when I hit a bump was a ton of rattling. I never got in the back and had my wife drive the car so I could pinpoint the problem. In short, the rattle was not the tailgate, but the rear shock mounts. Both were shot and the racket was unbelievable. Up until that point of reality, I thought it was the tailgate. The broken mounts were smacking up against the rear cargo panels. It might be worth checking out.

This is my first 850, I've driven the RWD Volvos for 20 years. And, I must say, I enjoy the feel of this car. It fits like a glove and the power and ride is lovely. As for the shock mount design....

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'90 245 DL 297K 91 944T 226K

My rattle was merely that the bottom metal latch had loosened and moved foward a bit. Where the rear door comes down there is an adjustable(slightly movable) part that the rear trunk door hooks onto.You see it right in front of you when you open the rear door and looked down. It catches a lot of dirt.
I loosened mine and moved it around and tightened( I had to try a couple places to find the right spot) it and found the rattle was gone. It is worth a try. Hope this helps.

There is a "kit" available at IPD or Ebay to put pieces of aliminum over the plastic so that the clips fit tighter. The dealer has the same thing. Simple fix required on all 850/70 wagons. Take the rear panel off first to verify you car doesn't already have it on there.
The rubber bumpers do need replacing. They usually get hard as a rock with age.

Klaus