I wanted to look up a few county routes, that are 600 series, but only 500 series routes are accessible, and this page comes up when you try to find county routes: http://www.state.nj.us/transportation/refdata/sldiag/Burlington.pdf (Burlington is an example, all 21 counties have this page)
You may have to expand the links on the left side of the PDF viewer.
after /sldiag/ type 01000601__-.pdf. Replace "01" with any number from 01 to 21, depending on the county's alphabetical position. Replace "601" with the number of interest. The first underscore can be replaced with "A" for an Alternate, "S" for a Spur, or "1", "2", etc. for a route with multiple pieces. (For example, Bergen County CR 62's eastern segment is 020000622_-.pdf.) The second underscore can be replaced with a "Z" if the route splits into a one-way pair and you want to look at the SB or WB roadway.