the slipper clutch is designed to slip when there is too much of a strain put on your drivetrain, i.e. if you are doing alot of jumping, limbing steep hills, or just straight out launching from a stop as hard as you can. This slips, so the stress put on the rest of the car doesnt cause you to blow out your diffs or strip the spur gear itself.

as for an idea what could be wrong, double check and make sure your clutch is in the right way, or that it isnt worn out. if its backwards it will slip instead of grip when there is a heavier load on your truck.