2001-early 2005 had 5 speed Allison 1000
2005 and newer has the 6 speed.
Allison 1000 Transmission Specs
As for the Dodge truck transmissions, I find that they don't like fluid changes. Believe it or not but out of all the Dodge trucks I've owned and/or serviced, those WITHOUT fluid/filter changes lasted the longest. Just keep an eye on the fluid level. Even our old 2004 Chrysler Pacifica stated in the owners manual that transmission/transaxle fluid did not need service under normal use. Only extreme use (limo or taxi) required 100k fluid change intervals.
At 120k on my 03 RAM with the 545RFE auto and I will not be changing the fluid or filter. It was only low once in it's life (@80k) and took less than half a quart. A good friend of mine once said auto transmissions are full of ATF and magic, once you open that oil pan the oil and magic come out. You can refill the oil but the magic is loss. :laughing: