Learn how to replace axle seals on a solid rear axle that is leaking gear oil. You will know the axle seals are bad because gear oil will drip down the tires and sling everywhere.
Luckily with this video you can see the process of replacing the axle seals. First you need to drain the differential. Next you need to unscrew the 4 nuts holding in the axle. Watch the video from ChrisFix:
Then you need to use a slide hammer to pop the axle out and now you have access to the seal. Use a seal puller tool to pop the seal out and then drive the new one in with a large socket or a seal driver tool.
Now would be a good time to replace the rear wheel bearing as well and to do that you need to cut the bearing in half to remove it and then use a pipe with a hammer (or a hydraulic press) to press the new one on.