Yeah, those rpms are correct.

2002 Dodge Ram 4.7L did have the 454rfe trans with final oputput drive of 0.75:1 The 5.7L had the newer 545rfe with 0.67:1 (12% reduction)

Let's compare:
My 2008 truck had 3:92's with 33" rubber and at 70mph I would turn right about 2100rpms with a 545rfe trans.

4:56's are 16% shorter of a gear which would make yours = 2436rpms IF you had tall 33" tires like I did AND a 545rfe trans. If you have the standard size 17" wheel, those are equip'd from the factory with 31.5" tires which would mean this bumps you up to 2552rpms at 70mph.
Lastly, I had the 545rfe which reduces final drive output by 12%.
at 70mph with above mentioned details and 4:56 gears for a 454rfe would mean 2856rpms would be correct.

Obviously there will be some differences here/there. I just wanted to give you a rough coorelation between the two.

 

There's always the option of installing a different TCM to turn it into a 545, it's been done.

 

Not possible. The 545rfe has an extra taller gear for overdrive. You cannot program the 45rfe for something it doesn't have. It's not a CVT transmission capable of that sort of thing. 545rfe is not the same transmission internally. Almost, but not quite.

 

If you dont have too many miles on the gears i might be interested in buying them from you.

 

3:55's will not get you best MPG if that is your intent. This has been discussed numerous times in the past. The 4.7L gets best overall mPG's with 3:92 gears as it keeps the tuck more in powerband. 3:55's you will constantly dog and dump fuel and go nowhere anytime fast. The older 4.7L's need the gearing as they are severely underpowered.
If you want to decrease your gearing, I wouldn't go lower than 3:92. If your 4wd, you'll need to do the front as well. I have this feeling you are 2wd tho.

 

I have a 4.7 in my Dakota, as for the 3.55 it was a dog, spent the money and went with a 4.10 set up. was a much need improvement and you would be very happy with those gears, around town I gained a mile or two but lost a mile or two on the highway, so its a give and take depending on what your driving mostly, around town or higway

 

The 45RFE and 545RFE are the same exact transmission other than programming. Its the programming that decides how many gears are used. The last time I replaced my transmission was a 45RFE, once my TCM took control of it, it becomes a 545. This modification has been done, give it a read:
http://www.dodge talk.com/forums/showthread.php?t=208132

Might want to read this too...
http://www.allpar.com/mopar/transmissions/545RFE.html