My 2 centavos which is intended to be in addition to the excellent points above...
Crawl under the rig three or four times in a week's period and soak the livin' bejeezes out of all the fasteners with penetrant.
Replace the 2 spring's center bolts.
Measure with a long enough string the distance from the trailer pintle to the exact center of the spindles on both sides. Make sure the string doesn't hang up on something in the middle. Distance doesn't matter, equalness does. Then measure from exact center front axle to rear axle.