Why will my van start but it won't Move?

Get a new fiance? Make him pay for new transmission if it's needed after leak is fixed.

Fixitmr is pretty smart but his computer isn't. It's rather difficult to see what's leaking from where we're sitting. That's a job for a live human being. Could be as simple as a popped rubber hose going to the transmission cooler in the radiator or as expensive as a leaking front pump seal. Given your description, I'd suspect a popped hose first. Regardless, continuing to drive it definitely caused internal damage to the clutch packs. Had he stopped right away when the slippage was noticed, repairing the leak and refilling with new fluid would have been the end of it. Instead, the loss of fluid prevents the hydraulic pressure from forming that is used to apply pressure to the clutch packs. That lets them slip and you lose speed while the engine speeds up. That slippage causes the clutch plates to overheat and be destroyed. THAT is what gets to be expensive.

If you're emotionally-involved with this fiance and don't want to replace him, you might look at this as an opportunity to trade the van for something else. Ford is having a huge problem with rear axles in them rusting apart. We'd hate to read that happened to yours after spending fistfulls of cash repairing the transmission.