You can find titles of Young's books by an internet search, then ask your library's inter-library loan dept. to borrow them for you, for a few dollars per volume.
Some militia rosters for York Co. should have been published in the 1880s or 1890s in the published /Pennsylvania Archives/, I think in the 3rd Series.You should be able to locate these listings on www.footnote.com (which has free trials available to view pages).
These published militia lists will have been extracted into card file indexes held by the Penna State Archives in Harrisburg.They have posted images of the cards on their web site.The gateway to the Rev. War and other military data extracts is here:
In the card file, the source citation is on the bottom; if from published volumes of the /Pennsylvania Archives/, the citation at lower right will give Series, Volume and page.In a great many instances the original records that were extracted and published no longer are held by the Archives, or are so fragile that public access is not allowed (they have not been microfilmed, either).
You already are aware that the Friends' Meetings often recorded when there were members who went astray by participating in militia activities (by law, folks such as Quakers and Mennonites were allowed to pay fines instead of showing up for drills/nose-counting).If you have not yet made intensive searches for such records, by all means do so.
The County Court records may also have entries concerning the County's trying to get members of pacifist religious organizations to pay the non-participation fines.These records must be searched in person (e.g., not by a member of the County Clerk's staff).It is possible that such Court records have been microfilmed -- you can search at www.familysearch.org in the Library Card Catalog for York County records.If you find some likely material, you can work through your nearest LDS Family History Center (which you can locate on the same site) to order the microfilm from Salt Lake City to look at.Some major libraries also have an agreement with LDS to borrow such microfilms as well.
All able-bodied free white males between ages roughly 16-60 (varied by time and place) were obligated to be available for active-duty militia service for defense within the county, and to show up for regular drills.Relatively few had active-duty service, which might include patrolling, guarding prisoners, or helping to escort supplies bound elsewhere.Surviving militia rosters were almost always written at the regular musters and do not reflect active duty.Very few militiamen were involved in active campaigns conducted by State Troops or the Continentals, but it did happen.