98 chevy astro front end vibrates and shakes at 65 mph

You may have to get your wheels re ballanced if you lost one of the weight's on the rim. The other thing to is a bent rim if you hit a large pot hole or crub and last to see if the tire it self has funny wear patterns on the treads.
Other things to look for if your wheels are ok. Check the ball joints Check the tie rod ends Check and pay attention to the Idler and Pitman arm. Your looking for any excessive movments
A good way is to grease all that is mentioned above and see if the vibration is gone.
Lost thing would be the brake rotors but you'ld now that by having vibrations throught out any speed when you apply the brakes to slow down.
Good luck and hope this helps, you should be able to do this on your own other then checking the wheel ballance. 

BUSHINGS! Your bushings need to be replaced. It is probably worse if you are in a turn at 65mph, too.

This particular van is being noted as having balljoint problems at around 100,000 miles. That would be the first place I'd have checked.

Had wheels replaced on my 2002 astro and balance vibration at 62
mphr mostly gone but when having oil change mechanic found
good lower ball joints and good sway bars but found play in my
left side wheel bearing.Will this stop vibration please respond.