trans shifting hard(ignore the other post plz(

i have an 88 s-10 4 wheel drive. 4.3. i just had to put a torque converter in it back in febuaray. now it is shift hard when goes from fist to second. then not so bad from second to 3rd. its still rough but not to bad. it just jurks the truck when it shifts and barks the tires. just wondering what u all think it might be. they told me to change the fluid at 6000 miles cause the fluid looked bad when they changed it. kind of dirty. i plan to change it. just want some ideas that might be wrong. it does seem the idling is turned up on it. could that have anything to do with it.

Its a good idea to flush the transmission and put new fluid and maybe a filter in it, but thats not your problem.

Theres a vacuum modulator on the back of the transmission. It pulls a manifold vacuum, and it sounds like the vacuum line has come loose. When this happens your truck usually idles higher, and it will shift later and harder.