rear hatch door closure

[filchuck]

I have 1997 Astro van. The rear hatch door will not stay open. I think the door closure pistons are not working.
Do they have to be replaced as a set, or can they be replaced one at a time ?
If so, how can you tell which one is defective ?
What's the proper procedure to remove and replace them.

Any assitance would be appreciated.

Thanks.