stole this from tjtj
The OEM shocks (00-04 X's)
Front
9" compressed/13" extended (4" Travel), 285/65 valving
Rear
14" - 23" (9" travel), 205/55 valved
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The Bilstein 5100's, or HD of same specs (00-04 X's), with what has come to be known as 20% stiffer valving....this does vary a bit across the line, but, is close:
Front
9" compressed/13" extended (4" Travel), 360/80 valving
Rear
16" - 26" (10" travel), 255/70 valving
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The second #, compression, helps to control the UNSPRUNG weight/movement/forces from the axles and tires, etc. (Stuff the suspension is not supporting)
The first #, rebound, helps to control the SPRUNG weight, like the frame, etc. (Stuff the suspension IS supporting)
So, as long as the weights and angles and so forth are similar, shocks for one rig can work fine on another.
Where it gets tricky is where the angles are off, like one truck has the shocks mounted at a 30º angle, and another has them at a 15º angle...
Because the larger the angle, the stiffer the valving has to be to control the same damping forces.
If my tires are 100 lb each, and yours are 50 lb each, well, I'll need more compression damping than you will, etc.
And so forth