The back of my 1981 300TD wagon seems to be riding quite low to the ground.

What might be the cause of this and how might I fix it?

I use the cargo area to haul percussion equipment, but even when empty, the car appears to slope downward toward the back.

Here is a picture of the vehicle from the side.

Your TD has a self-levelling rear suspension, consisting of one hydraulic pump and reservoir hopefully under the hood, two rather expensive hydraulic shocks over the rear axle, and a pair of air/hydraulic spheres that compensate in such as way that the rear end is supposed to remain level, regardless of the load, up to the max load printed on the rear gate threshhold.

Do a search on 300TD rear hydraulic and you will find a trove of information.
You will need to replace some special hydraulic fluid at least. The spheres cost about $79 each. The shocks are more expensive, as is the pump.

good luck, have fun

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)