To try and determine if it is the front or the rear, take your hands off the steering wheel when coming to a stop and look at the steering wheel for any sleight shimmy motion and/or truck pulling to either side.
If yes, the problem is in the front.
Another way to get a possible lead is to use a lazer heat temperature gun looking at which side gets hotter after a hard brake application.
If detected to one side, everything has to be closely looked at.
Is the caliper pads wearing excessive or at an angle?
Is both pins sliding easy in the caliper?
Is the caliper piston sticking when it heats up?
Is there signs the disc is showing friction at one area of rotation? disc appearence may show it.
Often it's temperature sensitive so do the rotation test when it's hot.
Sometimes a cast iron disc will harden from heat in one area while the rest wears creating a high spot and shudder is the result.
A warped disc, if not servere often won't show shudder because the caliper moves with the disc change as it's temperature changes during brake use.
Sometimes it's possible to distort a disc with excessive lug nut torque.
Remember the front pads are against the disc with a small amount of pressure at all times.
With emg brake shoes in the center of the rear discs, the emg brake hardware could be locked a little tight and if the surface is warped out of round, could cause shudder same as a full drum brake can.
A way to tell the rear is doing something is to raise the rear wheels off the road and run in gear. Application of brake shakes the rear and/or the whole truck, tells the story.
Good luck.