Welding a spindle properly onto the axle requires removal and placing it in a jig to get proper alignment. It can be done by guessing the alignment on the side of the road but this will only get you on home. The first thing you will notice is the tires wear out very very quickly!! Depending on how out of alignment it may be, you could scuff a tire beyond use in only a few hundred miles.
A few years ago Lippert had axles that were sent out of the factory with improper alignment and several folks had major issues with tire wear. I have experienced this and also lost a bearing on a large boat trailer that required spindle replacement. There are a few alignment shops that can handle welding on a new spindle properly but be very cautious about who you get. It's a very critical part of tire wear and few people can do it properly. Axles are not that expensive nor are they hard to replace. I would just replace the entire axle. There are several places on the Internet where you can buy axles at a reasonable price.
