As long as the read of your car does not bottom out on the top of the Ramp you can use them on the Rear.
As other have said it is better to have both Ramps and Jack Stands so you have a choice.
The Jacks stands are cheaper and allow more room for you to move around when you are under the Car.
Getting up on Ramps can sometimes cause problems. I have had one Ramp move while the other did not. I have also driven too far and off of them.
Ounce the car is up on Ramps and the Wheels are well Blocked/Chocked to prevent any chance of roll back Ramps are safer than Jack Stands.

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84 300D, 82 Volvo 244Gl Diesel