2000 silverado w/ 4l60e. Just had the trans rebuilt with upgraded clutches, 3/4 and band and trans go shift kit. I didnt do it a shop did. My problem is with 2nd to 3rd shift its a delay with a quick rise in rpms ??? Any ideas

 

This sound like a "flare" during the 2-3 shift. Immediately after a rebuild it is likely caused by a leak in the 3/4 apply circuit. Ask if they replace the 3rd accumulator check ball cartridge. A cut seal on the 3/4 piston can cause this, or looseness between the input drum and input shaft, or a bad seal in the servo, or....
This is really the shop's responsibility to solve it.

 

I was told it might be something in the valve body like a check ball was left or something. Only flares if im getting on it. Normal driving shifts fine idk im lost amd need it fixed !

 

Nope. A missing check ball will give you extremely firm shift into one or more gears. A small leak in the 3/4 circuit will only/mostly be evident during WOT shifts.
A bad Valve body (VB), e.g. stuck valve, could theoretically cause such problems, but based on other similar threads, it is less likely than a leak in the rebuild.

 

I know the valve body and valves and shift solenoids and shift kit worked awesome was removed from one of my previous transmissions. Never had a problem with it. So i guess its a leak somewhere.... This sucks. I was hoping it was something simple but that never happens.

 

The shop should first hook up a pressure gauge to see what the line pressure is during the shift. If the pressure is low, there might be a simple fix such as replacing/modifying the pressure solenoid, or swapping in a bigger boost valve.