After shifting seamlessly through 1-3 gears, it takes the transmission about 15-20 seconds at 2500rpms, or roughly 55 mph to finally shift.

I took it to Certified Transmissions, and they ran the transmission through the gauntlet and determined it was definitely NOT a transmission issue.

Does anyone have any suggestions on where the issue may lie? Thanks for your input!

is this on your van? im no trans expert but given all the common issues with that trans with slow shifts i would get a second opinion. find someone that has a bunch of 4l60 experience.