The SXT is really just a slightly enhanced ST and does not come with a third row seat standard, but can be ordered with one as an option. It's offered as a 2wd or 4wd, and with a V6 or a 4.7-liter V8.
The SLT adds body-colored bumpers, chrome side mouldings, larger alloy wheels and tires, a standard roof rack, woodgrain trim in the dash, much more plush cloth on the seats, and of course the standard third-row with rear air.
There is some overlap in the pricing which allows you to get a Durango SXT with a V8 and third-row/rear air for the same price as an SLT with a V6. To me, the SXT is the better value there.