1994 E150 transmission problem

hello everyone, my experience lies in mustangs, but my brother has an e150 conversion van with a 5.8 engine. He says when driving at highway speeds, the trans seems to slip out of gear, and the engine revs before it slams back into gear. Its had a recent fluid/filter change. fluid levels are normal. i'm thinking lockup converter problems. Anyone have any experience with one of these that could enlighten me? Thanks, Darren