I just bought a rear window WITHOUT SLIDER off Ebay for my 2004 Screw. How hard is it to replace? Is this something I should just let a professional do?

 

without experience i would let the auto glass place do it.or my favorite body shop.some speical tools make this job easier but no experience can cause chips or scratches that may lead to rust or leaks . you could also break one or both windows.

 

You will want to do a search on this site how to remove the backrest of the rear seat. It will be necessary to remove this to take out the rear window. You will not be able to remove the backrest with out having instructions on how to do it, even then it is difficult the first time.

Replacing the window also looks tricky, I'd recommend going with a pro after you removed the backrest for them.
Good Luck