Well I tried everything I could think of to get the diff plugs out to do a gear oil change. The drain would go with a 1/2" breaker bar but the fill plug was not going to budge.

Today I stopped off and picked up a long 17 mm combo wrench at Sears and got home. Then while the diff was still hot I threw the rear end up on some jack stands and put the new wrench to work. With the rear end hot the plug took one good jolt and it was loose. So there is another trick to getting that darn plug loose. Just run it till it's nice and hot. Works great.

Mine is stubborn as well. I always need to use a pipe over the wrench to get enough leverage to loosen the fill plug. I push on the left rear tire with my foot to prevent feeling like I will push the car of the stands.

you may be able to use gravity and geometry:

lift car

connect a pipe/wrench/bar so that one end is on the plug and the other on the ground in such a way that lowering the car turns the tool

lower the car

you can also use a block or jack stand to drop the tool handle on to if the ground is too far away

You can also use one of my favorite tools -- a big rubber mallet. Just bonk on the end of your wrench handle in the desired direction. You won't be able to bonk your car off the jack stands, but you'll get quite a lot of torque out of a good bonk.

Tried that which normally works for me, then went to the hand sledge and it still would not budge. It took the combination of heat and the long wrench that did the trick, but I think it was the heat.

PROBABLY heat, yes. Aluminum expands and shrinks faster than steel with temperature fluctuations, so the steel plug in the aluminum hole should fit more loosely when they're both hot.

I broke a 17mm wrench trying to get my new transmission fill plug off.. i snapped it right in half with a 3 foot pole on the end for leverage... I wish i had a torch at the time.

Needless to say, we gave up and just flipped the trans upside down, filled it through the drain hole and then kept it level during installation

Let's not forget our friend the backup light switch, right next to the fill plug, that is usually MUCH easier to get out.

Air impact wrench and a torch around the plug on the case, say no more. Use a 5 wrap of teflon tape on the install to prevent future binding, and never overtighten fill and drain plugs.

Air impact wrench and a torch around the plug on the case, say no more. Use a 5 wrap of teflon tape on the install to prevent future binding, and never overtighten fill and drain plugs.

Ditto