'95 w/95K miles, New Issue...

This morning after starting my van and getting on the road, a new problem has arisen. When I take off from a standing start, the van is lacking a bit of power (almost feels like a brake is dragging- they're not, I made sure). It feels like the van is trying to pull 10,000 pounds of weight. I only drove it 2-3 miles, then headed back home.

After test driving it in my neighborhood, the van doesn't shift out of first gear until you step the throttle down at least 1/2 way, then it quickly upshifts into second, but not harshly. I would say it upshifted too early, if anything. It's hard to say as I don't want to push it too hard and make anything worse.

I replaced the O2 sensor this morning as I got a "13" SES code and the SES light came on yesterday, but replacing it didn't change anything, other than eliminating the 13 code.

I've never had any trans issues since I bought it in March. Trans fluid level is correct. Fluid and filter were both changed two months ago with the correct DexMerc fluid. No signs of overheating, no hard shifting- no sign of bad things to come.

I read about the 1-2 shift solenoid issues, but I'm not sure if this qualifies, as there's no hard shifting.

I had the TC unbolted Sunday when I was searching for a persistent rattle, and now I'm wondering if I didn't index it properly. I marked it before I removed the three bolts, but I can't honestly remember if I matched it up again when I bolted it back to the flexplate. Does it have to be re-attached the way it was positioned before I unbolted it, or does it not matter?

Suggestions welcome.