You can do that in the descr_rebel_factions.txt (also unpack)
You'll see:

Code:
rebel_type			Livonian_Rebels
category			peasant_revolt 
chance				1					
description			Livonian_Rebels
unit				Woodsmen
unit				EE Peasant Archers
unit				Lithuanian Archers
unit				Viking Raiders
unit				Lithuanian Cavalry
Change the chance to any number lower (or higher...) than 3. Where this number is multiplied by 5. 0x5 = 0 for example :)
Not sure if you can make negative unrest, but I doubt it.

Change it for all entries btw (except the very first two above the example I posted)