There is 2 ways to re-grease them.
Remove the Boot Retainer Spring and apply Grease all over the Shaft and install the Boot. That will push the Grease up.
Do as above but fill the boot with Grease.

Leave the boot installed and get a Meat type Syringe or a Needle for your Grease Gun and slide the needle parallel to the Ball Joint Shaft and under the Boot and inject the Grease inside it.

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84 300D, 82 Volvo 244Gl Diesel