The shoes on my FEL plow dig in when the drive is soft too. I think someone on here added guide wheels, but I don't think they would be much better than shoes for me. Some others have added a slit pipe over the edge of the blade. I also saw some examples of pipe or solid bar mounted behind the blade edge. I'm looking into a variation of that, by welding 2 vertical 1" rods 10" long to the top edge of a 2 1/2" or 3" pipe. The rods would be spaced so they will insert into the existing shoe mount brackets on the back of the plow. That would give me a 90" skid behind the blade. The pipe would skid over the gravel, with the weight ditributed along the full length. I could tilt the blade forward to lift the skid above the cutting edge, and tilt it back to use the skid. I could use the height adjustment spacers from my original shoes too. Since I don't weld, I'm getting a price on the skid from a shop now. The price might be too high for me to have it made, but others could use this idea if they have scap metal and can weld.
A better design yet would be a rolling pipe with end posts to mount in the skid shoe brackets. Think of a full length anti scalp roller like mowers have.