Guys, I need to clarify the following 4WD maintenance items:

1. Re-torque drive shaft bolt
- how is this done?

2. Replace rear differential oil
- a few months ago, I had the rear oil seal fixed * autotechnika then had the mechanic * speedyfix to top-up the "gear oil" near the front of the engine (he had to lift the car to access it btw). is this the rear differential oil or the transfer case oil?

3. Replace transfer case oil
- how is this different from the rear differential oil?

3. Replace front differential oil
- is this connected with the rear differential oil?

Thanks!