I just had my speedometer gear fixed in my 88 iroc b/c the speedometer and cruise didnt work but when i got it back it ran horrible but the speedometer, and the cruise worked. The car wanted to shift into OD way way way too soon causing the RPM's to drop tremendously so i tried running it in just drive and it still shifted soon. I think it might have something to do with the TV (detent cable) but im not sure so if you guys could help that would be great.

 

Try adjusting the TV cable and see if that fixes it.