'91 s-10 blazer vacum problem

my '91 s-10 blazer will lock in the front end when accelerated hard or when I reach a speed of about80km/h (50mph) It will unlock when the rig is almost at a stop or if I put it in 4 wheel drive and back to two wheel drive. I've been told it might be a vacum switch, and when the truck is making vacum (acceleration) it kicks in and when simething takes vacum (the power booster from brakes) it kicks out. If this is true where is this switch and how can I trouble shoot this problem?