A $2 billion bid for the Pentagon’s primary rocket maker is running into complications, partly because Boeing Co. and Lockheed Martin Corp. differ on whether to sell their equal stakes in the joint venture, according to people familiar with the details.

Aerojet Rocketdyne Holdings Inc.’s bid for United Launch Alliance LLC, the joint venture, faces a situation in which Lockheed Martin’s executive suite seems more eager to negotiate a sale as part of a broader, companywide portfolio restructuring, these people said. On the other...