Why is my traction control light coming on?

Unfortunately diagnosing this usually requires reading the ABS module codes, which unfortunately most OBD-II readers can't do. Unless you know someone with a professional quality scanner that supports ABS modules, you may have to take it to a mechanic or a dealer that has the right equipment.

Not sure how much they would charge for the diagnostics, but once you have that information you would be in a better position to figure out what's wrong with your minivan.

The most frequent issue I know of is with the wheel speed sensors; since there's one on each wheel they're down there where they're susceptible to all kinds of environmental disasters for both the sensors themselves and the wiring leading up to them. Generally when one goes out you'll lose ABS and traction control; I'm not sure what might be going on with the shifting or why that would be associated with the problem - that's a new one on me.

But getting the codes will help pinpoint the issue; if you are indeed having a problem with a wheel speed sensor the codes will show that and even tell you which one it is. There are ways to test the wheel sensors with a voltmeter; I'll have to leave that as an exercise for the reader - there is plenty of information online on how to do that which can be found with a Google search.

That's where I'd start; if it's not a wheel speed sensor then it could be a bad wiring connection somewhere, which can be a nightmare to track down.