As far as I know nothing is different between those two years. Love those Dakotas and Durangos.
fwiw, The SLT would have a 5.2, so that motor isn't the original.
SLT was just a trim level. 99 Durangos were equipped with the 3.9l V6 as standard and the 5.2 and 5.9 were options. There weren't many V6 Durangos sold, so the 5.2 and 5.9 ones were the most popular and readily available. I've seen a couple SXT (base trim level) with V6s, but every SLT I've seen has been a 5.2 or 5.9. There was also a SLT "Plus" trim level that basically just added leather and premium sound (Infinity).
