I've been working on trucks and trailer for awhile now and I'm kinda curious as to why no one has ever put a rear end on the trailer axles? Wouldn't it save your tires and also eliminate the need for a dump vavle?

I take it you mean a differential when you say "rear end"? If So you don't need a diff or any gears back there because the tires are already free to turn at different speeds when cornering. I think you're thinking that both sides are turning on the same shaft but they're not, they are on individual hubs.
Some time watch a trailer when the driver is making a hard U turn, the tires on the right side may be going forward while the left side can be going backwards.

I missed the part where you said "dump valve" so I assume your talking about a spread axle? Ifso the tire are being dragged sideways in a hard turn and even being on separate hubs that doesn't cure the problem nor would a diff, What might would be a castor type arrangement kinda like on a shopping cart or the front lift axle on those heave trash hauling trailers.
Feel free to perfect and patten a design and make a million.

Could be a weight thing. Rear ends weigh a heck of a lot more than dump valves. More weight on the equipment leads to a lighter load and less of a payday. It may seem small but for bigger companies it adds up real fast on lost revenue.

Thank you I didn't know the tires were on separated hub assemblies. Which means a diff. would make no difference. Thanks for the input