2001 B2300, replaced thermostat and now truck is is idling really rough like its about to stall. Any ideas what the issue could be?

 

If you disconnected the battery then it has to relearn the idle. If the throttlebody is dirty then it will have to be cleaned because the PCM has set it to factory specs and there is not enough air going by the flap to hold the idle up.

 

Make sure that you didn't dislodge a wire or something when you did this.

And of course make sure all the air is bled out of the cooling system.