I just hit 18000 miles and the book says to "check" axle fluids and change if I'm a taxi or tow a lot etc. I don't do any hardcore wheeling, do I need to change the fluid?

If so what weight. I know on Project-jk says to use 75-90, but the dealership says 75-140. Does it make a huge difference?

 

I like to change diff oil after the first few thousand mile after new gears (factory or replacement gears) to purge the metal from the gears lapping in. So, go ahead and change the fluid. It's time.
80W-90 API GL-5 synthetic for front and rear, but 75W-140 synthetic in the rear axle if you tow.
I used Mobil 1 75W-90 in the front, and 75W-140 in the rear on mine.

 

That is good advise. I also like to remove the cover on the first change and clean a little to make sure most of the gear gunk and metal shavings are out. After that I don't remove the cover unless I have to.

 

For 2007 the requirement was to change it at 15K, not sure why it is different for you, but since it's relatively cheap and easy to do, I would change it. If using standard lubricants then use the required 75-140 (you might have the tow package). The Project-JK write-up involed Synth fluid, which explains the 75-90 oil.

*Don't let the dealer charge you hundeds to do this, it is easier to do than changing your motor oil. Since you have the LSD, you may need an additive if you stick with standard oils, if going Synth then an additive is not necessary.