[2000CAYukon]

Did you replace the fuel filter? Have you checked fuel pressure?

Not sure if this is the same issue but maybe it will help you. My 90 GMC started to take longer to shift into 3 or 4th gear. It was not slipping, it just took longer to shift into gear. I changed the fluid and filter but no change. So I started to research what it could be.

For me it turned out to be the seals in the servo (especially the 4th apply servo). I spent too much money on billet servo but I think the vette servo (around $20) and some new seals on the 4th apply servo would have been fine for my truck. All of this can be done without removing the pan!

It now shifts much better.