Praise, there was a thread about the various bumper shapes many months ago, but I can't seem to find it. IIRC, there's a noticable difference between the front and rear p/u bumpers; panel truck bumpers are harder to distinguish.
About your different shaped bumpers: When I had the Juggernaut in pieces, I noticed that the rear bumper for my panel has tighter curves than the front, and is just over 1" shorter. As a result, the rear bumper would not fit the curve of the front splash apron and the tips of the bumper would touch the front fenders. The rear splash apron under the barn doors curves sharply toward the back corners of the body, just like the bumper.
