Is there a mathematical equation to figure out what gears you have by speed and RPM? I looked on the rear axle to no avail, and unfortunately, I no longer have the sheet that listed all the features with my truck. Can someone please point me out to some type of formula possibly using 127 mph @ 3k rpm? Any suggestions or ideas would be greatly appreciated...

try looking in the glove box. there should be a sticker in there with what options you have.

call any dodge dealer give them your vin number and WALA

It would have to be in 3rd gear (1 to 1 ratio) and you would need speed, rpm and tire diameter to do the math.
Go to dodge.com, on the right click on "owners" and look for contact us. Shoot them an e-mail and request a build sheet for your truck. Include the complete vin number in your request. I got back all 3 of mine (3 different vehicles) within 24 hrs of the request.

At about 2000rpm in OD, mine is about 70mph. I have 20" wheels/tires and 3.92 gears.

Failing all that, jack up the rear end and turn the tires, counting drive shaft revolutions. It'll come out to 3.5, just over 4 or 4.5.