I had the same wear problem with the same boat ('99), trailer and original Tracker "bias" ply tires. I had the axle replaced because the dealer determined it was bent. In this case "WRONG ANSWER"!! New axle, new "bias" ply tires installed (not Tracker brand) = same uneven wear problem starting again in about 2500 miles.
Changed to radial tires and have not had any signs of uneven wear in about the same amount of miles. A friend of mine bought a new 2001 pt 175 and just replaced his tires this spring for the same uneven wear problem and his was far worse than mine for wear, but he had about 1/3 as many more miles. So far he is not noticing any wear problems.
If you search this website for tire problems you will see others have resolved this issue by getting rid of those "bias ply" tires and switching to trailer rated radials. Carl