Plates that mount to the front axle that corrects for caster after lifting the truck.
The 2.5 version corrects caster for a lift range from 2" - 3" and works with the stock front drive shaft.
The 4.0 version corrects caster for a lift range from 3.5" - 4.5" and works with a Double Cardon front shaft.
To calculate your ride height, measure