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It's hard to tell without looking, it could be just about anything. It could be that one (or more) of the axle mount bolts is loose (been there, done that) or one of those rubber isolators is shot. But it could also be in the suspension or steering (been there, done that, too).
The best thing to do would be to get the weight off the front end and then push/pull with your hands on the tires at the 12 and 6 o'clock positions and then at the 3 and 9 o'clock positions. While it's up on jackstands, crawl under and take a peek at the various suspension bushings, you might find something obvious. The tie rode ends were crappy, so if they haven't been replaced, give them a good look.
The only thing that looks bad to the eye are the control arm bushings. the van has around 112k on it and there is some wear on the inside of both front tires, more noticable on drivers side where the sound seems to come from.