The rear end on this truck has been changed whats the easy why to discover what i have?
You can jack up one wheel and count the revolutions of the drive shaft and get a pretty good idea (not exact) of what you have.
In order to do this, chock the front wheels, jack up one rear wheel, release the emergency brake and place the transmission in neutral. Mark the tire so you can count one revolution, then mark the drive shaft also so you can count the revolutions. With a helper, turn the driveshaft counting the revolutions, until the tire turns one complete turn.
Now to calculate the ratio, divide the revolutions of the driveshaft by two. As an example, if the driveshaft went around 8-1/4 times, divide 8.25 (converted to decimals) by 2 and you get 4.125. That would most likely be 4.11.
Disclaimer: If you have a posi-traction rear, you will need to jack up both wheels and you don’t need to divide it by 2. The actual revolutions of the drive shaft needed to turn the tire one turn, is the gear ratio.
If you need to know the exact ratio, you will have to pull the rear cover. Some gear sets have the ratio punched into the edge of the gear. If not, you can count the gear teeth and divide the larger number by the smaller number.