I've come across some mentions of US 86 being a preliminary number for US 84 in Alabama, including http://www.ajfroggie.com/roads/alroutes/alus.html . I found it on a 1936 Santa Rosa County map: http://research.archives.gov/description/5829759
This is now SR 87 south of Milton, but carried US 98 when that route was first designated. It could be a typo; other counties' maps show 98. But it could have been part of the plan, had 86 turned south to Mobile and east to Pensacola.