The pins you picture are an earlier version of caliper side and retention hardware. It irritates me that the fronts on E450 are the newer slide bolt setup and the rears are this old design.
From what I see here I believe something is sticking. Those pins wedge the calipers into slide grooves in the caliper mounting bracket. Either those slides, the caliper surfaces or the pins are dirty/rusty, and/or...
The caliper pistons are sticking. Either or both can leave the pads in tight contact with the rotor. Tight contact will cause damage to the pads, the rotor or even the caliper itself from heat of friction.
I think you need to find out what's sticking and free it. This could involve rebuilding the calipers or replacing them. Change the hoses too. The pads are shot, even if they still have thickness.
