Raise the vehicle on jackstands.
This is a given and nobody should have to tell you to do so. Working under an unsupported vehicle is for morons!
Raise the rear of the vehicle and support it by the frame with jack stands. Do not support it by the differential.
Support the differential with a jack and remove the rear shocks. Save the nuts, bolts, and washers in a zip-loc bag and label it 'SHOCKS'.
