Sounds to me like the vacuum pulloff switch for the engaging of the 4WD. I had this happen on my 1991 Blazer. The valve leakes and sends vacuum to the pulloff and attempts to partially engage when not asked to. Check the vacuum switch to see if it holds a vacuum. A new one is around $30. It is located on the transfer case with 3 vacuum lines running to it. This is a very common part to go. Also check all of the vacuum lines in this series to see none have leaks. Some are routed near the exhaust and the lines can get old, dry and cracked. If you need more info email me at
[email protected]. I can give you more info. Good luck.