1995 G30 chevy van "drifts" on the highway.

replace the worn steering parts and have a wheel alignment done

Worn steering linkage or worn c/v shaft

You could have some bad control arm bushings or inner or outer tie rod ends.

this could be worn out steering parts or steering box even supenshion bushes or broken springs will do this, jack it up and look for play in the steering parts

Could be any number of factors, steering box having too much play, bad steering parts, bad tires, out of alignment

On my 91 it is under the dash - just to the left of the steering wheel column.

they un bolt under the vehicle

Something is wrong with the front end. Bring it in for a front end alignment (that could be all it needs) and have them check for problems with front end suspension components. There may be something that is worn out ( such as ball joints, or tie rod ends) that is causing the uneven tire wear. There may also be a problem from hitting a large pot hole, or curb, which bent something causing this. Tell the shop to NOT align the front end if they find that some components need to be replaced, as when something is replaced, you should have the front end aligned, and you don't want to have it done twice.

If vehicle was left outside or in an unheated garage, there is a strong possibility that condensation has contaminated your fuel. Easiest way to check is to remove fuel filter and pour contents into a glass jar. If you see two different layers of liquid in the jar, the bottom part is water. Replace with new filter and drop the tank. Remove sending unit. pour contents into a large container and clean out inside with clean dry rags. Remember, fuel is dangerous to work with and fumes travel pretty far on the ground. So, no open flames electrical stuff, cigarettes etc for a good perimeter around work area. Generally, you can re-use top 3/4 of fuel you removed by syphon. Don't use remainder in your vehicle. Part can be used for lawnmower etc as long as you only take the part above the water line.

Find the power steering pump. The fluid goes in the top of the pump. Look for a twist off cap that has a measuring stick attached to the under side, when you take it off. Make sure you use the correct type of fuild for YOUR kind of vehicle. There are diffrent types for different makes of vehicles. Don't OVERFILL the pump.