I've been there. Have a '95 Windstar and a 3500lb boat. Thought I could get away with towing it but, it was no fun. The van worked very hard on the hills. It was the main reason I bought the truck. I think the Windstars are rated 2000 lbs towing and 3500 lbs with the optional tow pkg. Also, the trans has been suspect in Windstars from the get go. It would be risky to abuse by towing greater than recommended.
If you get anything bigger than a pop-up, consider a different tow vehicle. A '99 isn't exactly a beater.

Anyone telling you they are towing 8K with the van is jerking your chain.

Good Luck,
Raoul

1999 F150 4x2 Lariat RegCab Shortbed
5.4L 3.55LS Tow pkg