What year is your truck, I can move this thread into the proper location.
Here are some simple instructions.
What gear ratio do I have?
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.
96 down has a sticker under the hood that lists it. 97-up have a sticker in the glove box that lists it. Or you can request a build sheet from Dodge.