Hard time shifting from 1st to 2nd

hello everyone,
Got a little prob with my tranny. I have a 87 burban and it just started not wanting to shift from 1st to 2nd. I have driven it with no problem. I just got back from the lake and it had no symptons at all. It pulled my boat just fine. I don't think the tranny is going out. My guess is maybe the vaccum valve or hose. You guys have any ideas of what it could be. I am prepared to replace the tranny if need be. Could it be the belt?

Thanks a bunch