2000 Chevrolet Blazer - odd shifting

When going down the road it will down shift and then it will shift into passing gear and then back to normal and repeat a little further down the road.

Okay… Your question? If you post back, I have a bunch of questions!

“Passing gear” is actually a downshift, so your description is unclear.
When you state “passing gear” are you actually referring to “top gear” or “overdrive”?

I assume that it’s shifting differently.
Do the easy thing first - check your transmission fluid level.

Could be a transmission problem. But assuming there’s no basic problem w/the transmission hardware, then it must be something wrong with its decision making. The transmission’s shifting algorithm inputs are primarily the vehicle speed and the engine load, so ask you shop to double-check the stuff that affects that. i.e. the vehicle speed sensor, throttle position sensor. If there’s an intake manifold vacuum sensor (eg MAP), that too. The transmission won’t work correctly if the engine is performing poorly too, so check for any engine performance problems, diagnostic codes etc.