An update... I took the transmission apart, and at first I didn't see anything unusual, broken, etc. As I was washing the internal one-way bearing, one of the little "leaf springs" that keep the rollers in place (see
http://www.rctek.com/info_bearings_one_way.html) fell out. Further inspection showed that all of the rollers were loose in the bearing, which was very different from the similar one-way bearing used in the external shift unit (all of its rollers were firmly held in place). Still, the one-way action worked fine when I tried as hard as I could by hand to get it to slip. I put it all back together and it worked fine (no free-wheeling) for about half a tank and then started doing the same thing as before. I tightened down both shift units so they couldn't shift and was able to run another couple tanks through in 1st gear only with no free-wheeling. My theory is that the internal bearing (which is obviously damaged) has a "breaking" point now, and as soon as the shift to 2nd happens, that point is reached, and it freewheels until it slows enough to grip again.