I am looking at a 99 F350 Diesel with 145k miles. The trans shifts great 1 to 2 and 2 to 3 then at times there is a slight slip going 3 to 4/OD. I see a 100 or so RPM increase and the slip is a second or less long. It this Tranny dying soon?

It sounds that way. I had a 93 that started doing that and it just get's worse.