Playing with the sound:
* itchiness - This is the most direct rhyme, emphasizing the uncomfortable feeling.
* scratchy - This is a more informal, but still rhyming, alternative.
Focus on the cause:
* roughness - This rhymes with "toughness" and might be a good fit if the pants are made of rough material.
Focus on the effect:
* rashness - This rhymes with "trashiness" and is a good option if the scratchiness is causing irritation.
Ultimately, the best rhyming word pair depends on the specific context of your writing.