Mike,
I had the exact same problem on my 95' 170TF. I got tired of going through tires and the local trailer shop guy ordered a custom axle from Florida, cost me around $250.00. The original axle on the trailer was rated at 1500lbs if I remember right, could be mistaken, but I know it was surprisingly low. The one I replaced it with is rated for 3500 lbs and is hell for stout. I also replaced the springs (minimal cost) and it rides like a whole different trailer, doesn't have the oscillating bounce the last set up had and the money is well worth the confidence and peace of mind on the road. Good luck.
Jim