It's impossible to give a precise location without seeing the engine, as the layout might vary slightly depending on the engine size (4.3L, 4.8L, 5.3L, or 6.0L were common in 2008 Express vans).
To find it, I recommend:
1. Consult your owner's manual: This is the best resource. It will have a diagram showing the dipstick's location.
2. Look at the top of the engine: Focus on the area near the front, where you see various engine components.
3. Look for a yellow or orange handle: This is the most common color for the dipstick.
If you can't find it after searching these areas, a quick online image search for "2008 Chevrolet Express 1500 engine bay" can help you visualize the general layout of the engine and where the dipstick might be situated.