I can visualize the problem you have as I had it once on car trunk mount that wasn't flat. I ended up bending the antenna slightly so that even though the mount was angled, the antenna remained true vertical when the lid was closed. If you have already drilled a hole in your fender for the NMO mount, I'm not sure of any other solution. I've never seen a swivel as you describe, but I guess you could always have one fabricated at a machine shop.
On my current pickup (2012 Dodge), I use a fender mount that is vehicle specific and involves no drilling. The one for your 2013 Ford truck is listed here - http://www.theantennafarm.com/catalog/ford-f150-2009-2014-ford-expedition-2009-2014-4089.html