My van is doing something similar at the moment as well... Firstly you would want to eliminate the tires as the issue... Unbalanced tires will cause a shake in the steering wheel at high speeds and be less noticeable at lower speeds. If you've had your tires balanced recently then you can assume it's not that. From there you could move into suspecting parts of the suspension. Worn tie rod ends , worn ball joints, worn control arm bushings, worn sway bar links or bushings, on 2WD vans there is also a positioning bar of sorts from the lower control arm to the frame that also has a bushing. You can jack the front of the van up and grab the tire at 9oclock and 3oclock and try and shake it back and forth, this will tell you if there is play in the tie rod ends (inner or outer). Grab it at 12oclock and 6oclock and do the same shake... this will tell you if there is wheel bearing or balljoint play. I think that i've narrowed mine down to worn sway bar links as I cant find much play anywhere else and they are quite rough looking on my van.. until I replace them tho i wont know for sure..

good luck