1994 Chevy G20 Van gears seem to be broken

I have a 1994 Chevy G20 van, and a month ago or so, my band went on a tour, and we shared our van with another band and also hauled a trailer, on our way home from the tour we noticed that our 2nd gear and 4th gear went out, the rpms get really high. The rpms get high and finally switch, once it switches gears it runs fine, but the 2nd gear and 4th gear seem to be out. What should I do? We need to get the van back on the road so we can keep rocking!