"Every time I've tried to have the fluid checked (by someone else) they tell me it's a sealed unit and they can't do it."

That quote just proves that you have yet to find a decent shop. At least you didn't pay them to do something they don't know how to do (i.e. check the fluid level when they don't have a clue). One can only imagine what kind of rebuild or swap such shops would do.

Anyhow as to how to check the level it is the FAQ - http://isuzufaq.ibctech.ca/ It is item #42. A search of this Board will no doubt also pull up plenty of other information.

Check that fluid then post back. Do it ASAP as these trannies do not like dirty or low fluid levels for any extended period of time.