You absolutely must determine which transmission type you have before attempting this. Looking at the shifter is the easiest way. A 4-speed automatic will have PRNDL positions, and a 5-speed manual will have 1-5 and R.
Once you know the transmission type, you'll need a repair manual specific to your Astro's year and transmission. This manual will have diagrams showing the precise location of the drain plug. Generic information online can be misleading and lead to mistakes.
Do not attempt this without a repair manual. Improperly locating or removing the drain bolt can cause damage to your transmission.
Your best bet is to obtain a Haynes or Chilton repair manual for your 1998 Chevrolet Astro van. These manuals are relatively inexpensive and will have detailed diagrams and instructions. You can find them at most auto parts stores or online.
