Volvo 850, V70 rear hatch struts get weak. My buddy's rear hatch was head hunting so I picked him up a set of support struts from the junk yard. It takes about 30 minutes to change them out but you have to be careful not to bust up the plastic trim.

 

I used the StrongArm 4138 struts on mine. I got them from eBay for around $25 apiece, shipped.

I like Strongarm. Good, quality product. Made in the USA.

I used a shower curtain prop rod to support my tailgate while I worked on it. I keep one around to use on cars that have missing/broken prop rods, or worn out hood shocks. It's nice, because while the originals sag a little, you can crank the rod out a little further to give the hood or tailgate a little extra 'boost' as the replacements won't compress down far enough to hook them on the mounting *****.

On mine, I had to loosen the bolts on the forward shock mount to get a little bit of 'wiggle room' when installing the replacements.

It makes a big difference. I'm no longer getting smacked in the noggin by the door - I pop the latch, give it a very slight lift, and the shocks open it the rest of the way.

Also, IPD and FCP have tailgate hardware kits that you'll probably need if your shocks were bad. The door slamming down does the original clips and mount points of the tailgate panel in. The FCP kit is like 8 bucks with new clips and mount reinforcements. Volvo also makes a similar kit, but it comes with a price...