If it is like the SD, it does not use vacuum to upshift. In fact you can disconnect the vac line from the tranny and it will upshift fine. Needs vac only for smooth downshift.

Check the fluid level. If it has been leaking, it will get to the point where it finally will almost refuse to upshift. I used to drive mine until that happened, turn around and drive home, fill it up with tranny fluid, and drive for about six months before it happened again. Very tough transmission; never hurt it and it always shifted perfectly smoothely after I put tranny fluid back in it.