Here's the scenario. I'll go out in the morning and start the Blazer. After the engine is running it feels like something in the transmission then engages. The rest of the day it's fine. This last winter we left it in 4 Hi most of the time and it didn't do it once. When we went back to 2 wd it started up again. Maybe a vacuum problem? I have no clue. It's been doing it off and on for about 2 years. I'm thinking I should address the problem before something serious happens. Does anyone have any ideas? It's a 2000 Blazer LT