I sucessfully changed the oil in my differential ('86 300 E).
Thank you all for your suggestions in helping me change my differential fluid. I sincerely appreciate it. This forum rocks.
This is what my friend & I did:
1. Used a 14 mm (regular) allen wrench. One of us held it in the hex hole on the filler plug while the other used a mallet to strike the free end of the allen wrench. The after a few blows, the plug loosened & came out.
2. We repeated the procedure for the drain plug too.
3. Drained the oil and installed the drain plug.
4. Filled the differential with Redline synthetic gear oil (75W90) using a pump. We inserted the tube that comes out of the pump into the fill hole and placed the the pump behind the rear axle (the tube that came with the pump was long enough). There was enough clearence there and in less than 5 minutes, we had the differential filled. (After pumping some of the fluid in, I cut the neck portion of the bottle and so the opening was wide enough for the pump's stem to go all the way in. After the first bottle was empty, I poured some oil from the second bottle into the cut bottle and pumped away).
5. Installed the filler plug.
6. Then, we drove the vehicle's rear wheels onto ramps. This gave us the required clearence and we were able to torque the plugs in.
Once again, thank you all.