Transmission Shift Issue

I have an very dependable 2000 GMC conversion van with 140K on it. Occasionally the check engine light comes on and immediately the automatic transmission starts shifting extremely clunky. I assume this is the indication of a vacuum issue somewhere but I don't have any way of reading the error codes. Anyone have a suggestion where I should look first?

You most likely have a transmission code being set and the tranny is going into limp mode (line pressure is increased to protect the tranny).

Most Autozones will read codes for free.

//2000CAYukon