I have a 2007 Z21 w/a 250 Evinrude HO on it. I have a 2016 tandem Ranger Trail Trailer with tire size 215/65/17 which was what was on it when I got it. I replaced all four tires with Goodyear tires and the front axle is having no issues. However I am getting fender rub on the back axle and there is almost no clearance in there. Should I go with a smaller tire or is there something more pressing going on under there? Thanks in advance. 

The fender likely has some adjustment up and down could also have a worn out spring. Something is going on for it to have been fine before with that size tire and now not have clearance. Also make sure they put the correct size tire on 

Goodyear Marathon tire is superceded with Goodyear Endurance tire. Check the tires to see if they are the same type.

Tom

I don't think it's a tire issue.  Might want to take into a trailer shop to take a look at it.

Is your hitch height too high? The trailer should be fairly level. 

Are the tires well centered in the fender well?

Look to see if you're missing a block on that side of the axle.