I have the exact same problem on my 1980's gold rush. and my 2014 ram 2500. Back then, when these trailers were built, the trucks didn't sit nearly as high. I have already damaged my tailgate, when the trailer 'arms' contacted the top of the tailgate, and also damaged the top back corners of the truck bed from the same problem. Also the back of the trailer drags horribly when the angle is severe. (I have to maneuver very uneven terrain turning it around at my house). I am much more conscious to also look at the distance between the arms and the tailgate top while maneuvering and not just look at the mirrors at where the trailer is going
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In my case, my trailer had 4-pack leaf springs. I installed 5 pack leaf springs and that gained 1" on the rear of the trailer. Better, but still sits very uneven (low in the rear). I am now looking into installing air bags on the trailer suspension. Gold Rush Phil mentioned that yes, some people have done this.
Chris Bowman, the lower hitch will get the trailer to sit level, but makes the problem I have with the arms now being even closer to the tailgate and the bed top rails....
Kp