you can but it is difficult the internal diff is right up next to the hole so fishing a hose down through the fill plug is hard and you could get it stuck inside, you may never get the hose to the bottom of the axle case either.
removing the nose isn't a good idea unless you know you do not have lapped gears inside.
So you have to crack the front of the 3rd member off the axle case then have to install a new seal and retorque everything. which is annoying.
i give you this option.
Removing one axle isn't that bad, and will give you access to the bottom of the case.
it is 4 bolts and one seal to pull an axle, you will need to pop a rear cylinder brake line to remove the backing plate. now this gives you access to service the drums and drain the axle case. you can either use a long hose and a pump or jack the car up on one side and lower the rear on the other to drain the fluid out, as much as you can. Granted this isn't the best option but the 3rd member is VERY heavy usually a 2 person job so you do not injurer yourself.
you can try the fill plug removal, but it will be almost impossible to get the oil out and you will not get all of it.
the best is crack the 3rd member open drain everything look inside and inspection and then reseal it.
popping the nose can work but if you have lapped factory gears you will be in trouble lining up the pinion and crown correctly without removing the entire rear.
going through one axle and draining that way could be more successful either using a pump or lifting the car at an angle to drain as much fluid as you can. however if there is mechanical damage inside the case it will stay at the bottom of the case.
for an open diff you do not need a friction modifier so you can use anything really.
i've used 9+ gear oil from, Currie 85W140
