Here's how you start
1) check the fluid level and correct it if necessary.
2) if the fluid is dark or has any stuff in it (from step #1), drain and fill the tranny or better yet, have a shop do a flush.
3) Make sure that the TV cable is adjusted properly.
This is all stuff that needs to be done every so often anyway. If it still doesn't work after that, you probably have internal problems. I'm not clear from your message whether you are talking about delayed downshifts or upshifts. On the upshifts, they should be delayed when you juice it, so I assume that you are talking about delayed downshifts. If the stuff above doesn't take care of it, it could be a bad TV down pressure regulator valve or maybe a bad governor. It doesn't sound like a clutch, band or servo since it happens in both 2nd and 3rd.
Did you get a warranty on your rebuild? It is very common for trannies to have to go back to the shop in less than a year for minor problems that develop. Most shops will give you a diagnosis/estimate for free, but it's good to get an opinion from a couple of places.
'85 JIMMY
'90 paint/grill/trim 5"susp 33/12.5/16.5
Fresh 350 B&M '87 TH700