I used to have 3 of those style cable jacks to unload the last panther camper shell I had. The shell only weighed about 700 lbs, but it was nerve-wracking to use the jacks. I never had an issue, but I did not have much faith in them. I was worried that if I bumped the camper that if would fall off the jacks. I'm not sure what other options are available, but cable jacks would not be my first choice.
How much too tall is your truck? Could you simply deflate the tires? Is the shell simply light enough to manually slide it off the back of your truck onto a slightly lower dolly?
Wherever you go there you are.