The axles will be evenly loaded because they have equalizing links. I'd wonder if the rear axle is a lighter weight rating. No telling what the trailer maker slapped in when your trailer was assembled. Check the outside diameter of the axle tube, but you can't see the wall thickness--5200# axles have a 3" O.D. and 3/16" wall thickness. With the trailer normally sitting on flat, level pavement, take a close look at the axles. Both axles should be straight and all tires should be straight up & down. If you can jack under the frame right near the wheels, lift both sides and take another look. Both axles should be cambered (bowed up in the middle) equally.
Trailer tires are so junky that I'd get all new tires after four years and serious wear on two of them. I'd also get ST235/75R15 Load Range D tires for an extra 500# of weight margin, or Load Range E for 900# more margin. I'm a believer in balancing everything that rotates. Many trailer wheels must be balanced using the lug-centric adapter on the balancing machine, because the wheel's center pilot hole is not in the true center of the wheel. Demand that the tire store balance the wheels centering them on the lug holes, not the center pilot hole (less work for them using the pilot hole). New valve stems always (and I prefer metal stems for reliability). Watch the tires as they spin slowly on the balancer...if they're kind'a egg shaped, reject them. Even when balanced they won't run smoothly.