Welcome to the crappy world of the Chevy Venture, the most problematic vehicle I have ever owned!

I would bet that the steering rack is getting near it's last leg. See if you can take the van somewhere and have the power steering system flushed and new fluid put in, remember this is a mostly forgotten about service just as the transmission fluid is too. Don't you just love where they put the power steering fluid resevior on this van. You know the reason they put the alternator in the back and the power steering fluid on the top so close to the wipers, don't you? It's because Chevy's never need new alternators and the would NEVER leak P.S. Fluid!!!
Matthew Brough
1998 Chevy Venture -- 89,000 miles
1988 Chevy Silverado -- 148,000 miles
1977 Chevy Scottsdale C20 -- 275000 miles - no rebuild!!! I love my old truck!!