You're darn right they offered the 5.9 with the SLTs!
You didn't say what year your truck is, but 3 model years old = 2001 or 2002, so your choices are a 4.7 or 5.9- the 5.2 was dropped after 2000.
Check the VIN on the door or dash- if the 8th character from the left is a "Z", you have a 5.9. If it is a "Y", you have a 5.2. If it is an "N", you have a 4.7.
Here is how I was able to powershop for the R/T:
VIN breakdown (?= can be several characters)
1= US assembled
H= Weight range
R= 2WD S= 4wd
?8= Model (some variation here by year, for example the SLT & R/T were both a 28 in '01, and the R/T was a 78 in 02-03. In 02-03, 38= base, 48= SLT, 58= SLT+, 78= R/T)
N/Y/Z= Motor as above
?= Check digit
?= Year (1= 01, 2= 02, etc.)
?= Plant (? if there is more than one)
??????= Serial #
If you are on Autortrader and want a 2002 R/T after you have narrowed your search to all 2002 Durangos, you can use the "find" feature of your browser to highlight anyplace the characters "78Z" appear, and that will be an R/T. If you wanted a 2WD SLT 4.7, you would do "R48N".