The reason for bumpstops is so that your axle articulation stops before you crush a shock or shear it off the frame.
To see if/how much you need to drop them, do the following:
1) With your truck parked, measure the distance from where the bumpstop would make contact with the axle.
2) Now measure the amount of shock rod you have sticking out of the body (basically, how much more travel the shock has).
3) Subtract value #2 from value #1 and that will tell you how big of a spacer you need to put in there. (You may want to add an inch to this value, just to take into account the rubber bumpstop compressing a bit).