How Important: Changing Transaxle Fluid

[1981_300sd]

Hey all, I am wondering how important changing the transaxle fluid is on our diesels. I found a 14mm hex tool, applied some lube, and had a cheater bar - with no success on removing the fill plug.

So I will probably either have to wait for fall (getting really hot outside already here in Phoenix to work under cars) or take it to a mechanic to change or at least loosen the plugs.

What are your opinions?

Thanks!