Q: How much gear oil goes in rear different on e150 v8 1999 economy van

Most differential housings (the pumpkin) will have a fill plug or a drain plug and fill plug. If it just has a fill plug, the only way to drain is to remove the cover, which it sounds like you have done already. If there is a drain and fill plug, you can just remove the drain plug to drain. When it comes time to fill back up, ensure that cover or drain plug is tight, then begin to fill through the fill plug. When the fluid begins to drip out of the fill plug, this means that the differential is full. You might also refer to your owners manual to ensure you are adding enough. Most rear differentials on these vehicles take between 1-2 quarts of gear oil.