Affirming NLRB finding of Section 8 violation where discharged employee, who was “union ‘spearhead’ for organizing the [c]ompany's drivers,” had been soliciting union support on day before abrupt discharge, and employer's asserted reasons that employee had poor employment record, had received traffic tickets, and submitted incomplete paperwork—including “a particularly serious incident ... that involved missing cash collections” for which he was warned—were contradictory and pretextual, and where treatment of other employees for similar misconduct was disparate