06 T1N Can
The shift from 2nd to 3rd, for me, when I'm taking it easy will normally cause a rumble strip noise right around 20mph. To avoid this , I would drive a little more aggressively (with more pedal to the metal) so that the shift happens above 20mph.I just attempted a fluid change on a 2004 with 348000. I had been having troubles with shifting from 2nd to 3rd. when the transmission tried to shift, it would go into 'neutral' before shifting. if I timed it just right I could just barely let off the pedal at the right rpms and it would shift a little smoother and I would continue driving. a few weeks ago it completely failed to go into 3rd, and I had to coast off the highway. I let it sit for a few minutes and got back home ok. I read the code, and it was incorrect gear ratio.
I'm wondering if it's similar but more drastic in your case. For me, if it's 348000 and you do not have a history of the van, I would plan on having it remanufactured or having it replaced with one. Fluid is expensive and would be a waste as it sounds like it will only be a bandaid.
What was the fluid like when you drain it? How was the old filter?
Sometimes at the mileage, it would have been better to top it off because a drain removes the friction material that has come off and are floating in the fluid. But, it doesn't sound like it since it's faster and smoother now.