Here is a screenshot of RVTowcheck - I think it might be the original online one but not sure.
As I remember there might have been several possible adjustments to be made to the final figure - you can check all that on his website.





I don't have any association with RVTowcheck - it's just that it was so well supported and explained that I'm happy to pass on link.

 

Ok, major correction here . . .
I see that there is no axle ratio entry in RVTowcheck so I went back to my notes.
What I'm almost certain I did actually, is use the manufacturer's towing guide, where you look for the line in the table of your model, engine size, drive ratio, and find the towing capacity.

Most auto manufacturer's publish a Towing Guide for each year, and I either used that or the actual vehicle manual - the tow section - which factors in the same info.
If I can find exactly what I used I'll write back, but I now remember it was the manufacture's published towing capacity, which is probably the way to go.
And I do remember it was the engine and axle drive ratio that made the most difference.

For your van : It looks like the same towing capacity data is available from TrailerLife - here's the link for the pdf.
Specific axle ratios are signified by a, b, c, etc.
2001 Towing Guide - trailerlife (PDF)

RVTowcheck is good, especially for all the supporting info, but in the end I used the manufacturer's info.

 

Looks like I'm a little late here with no new info but just wanted to ask if you have a tow button on the dash? My 2007 was out of a commercial fleet and had one. But, my other 2 gvans did not and unless it's a very light load, I've always been told not to tow anything in overdrive. Possibility of west Texas being a exception. Deflated 2c tossed in