a short update:
so i've been continuing to drive the van lately regardless of the problem, but now when the speedometer acts up, the transmission behaves differently. not as bad as before the rebuild, but it doesn't quite know when exactly to shift.
i'm going to keep the van parked for a couple of days and replace the vehicle speed sensor when i get a chance later this week and see if that does anything. i'm starting there because it seems logical, and the part is cheap. i'll post again with any updates after that. | 
