My trailer had springs sized as Spike99 elaborated. 5000# across two axles, 1250# per spring. GVWR 5345#. I was headed out on vacation last summer and got 90 miles from home. Pulled into a rest area for a sandwich and did my walk around. Something didn't look right. Left rear trailer spring was broken! Managed a tow to a shop, overnight in a motel and back on the way next morning with a new spring.
Got home and did some research. Apparently Lippert had some bad springs, but these were loaded just below ratings (I scaled the trailer loaded). I ordered four 1750# springs and changed them. The other rear spring was considerably flatter than the front ones. Apparently the rear axles are more heavily loaded (I'll have to rescale the trailer to verify this). New springs installed and thing pulls much better. I had already done the Dexter upgrade with the EZ Flex.
I will be watching the springs and axles closely this year. The EZ Flex with the heavier links and wet bolts make all the difference in the world. Smoother ride, and absolutely no spring creak.
If you haven't, it wouldn't hurt to look at your axles and springs. I was lucky the spring didn't break at 65mph. Apparently it did it as I stopped and there was no collateral damage.