4WD Vacuum line diagram?

That wouldn't be the vacuum lines. All the vacuum does is move the front axle interlock in and out. Basically you move the interior lever and that slides a gear inside the transfer case from 2 WD into 4 WD or 4WD Low (which engages a set of planetary gears), In both positions the internal lever in the Tcase just bumps a plunger that opens a vacuum line to the axle actuator and locks the axle. When it moves the shift yoke in the interlock it also moves the switch that turns on the 4WD light. When you shift into 2WD high the plunger moves and takes the vacuum away from one side of the interconnect and applies it to the other to unlock the front axle and turn off the light, while venting the switch. For 4 low to operate the interlock has to move the same way as it does in 4 high. Are you sure 4 low is engaging? Does the 4wd light come on for both low and high? When you say it "slips" what is slipping?

Check over the brakes, especially the parking brakes. Could have moisture in the cables that froze. That would smell like clutch and create a lot of drag.