If you want to be 100% sure, you will have to drop the diff cover and take a look. I know that the gear lube and RTV will probably cost about $10 and you will never gte that 30 minutes of your life back, but it just might be worth the sacrifice
Here's what they look like
The top photo is a trac lock. Note the shape of the case. The arrow points to where you will look for the clutch packs.
The lower photo is an ordinary Dana open diff. Note how the case is more rounded on the non-ring gear side.