[GWN7]

To determine if it's "pooched" you need to remove the remains of the old bearing and wipe the axel down to remove the road grime and I'd say old grease, but there doesn't appear to be any. Then check the axel (which should be nice and shiny) for any scoring. If it's scored (depending on how bad) you might have to replace the axel.

As johnco mentioned yes you can replace just the end but you need to take the axel out, cut the end off with your metal cutting band saw, then place the axel in a jig to hold the new end and axel completely level with each other and then weld them together. Your welding skills and welder up to that job? Failure to get the axel and new end level will result in a trailer that will wobble dangerously when being towed at highway speeds and tire failure.