Definitely fixable, but whether it is worth it will be a question that depends on the rest of the truck. How is the frame in general? Seeing this bracket and the third member, there may be too much rust all around to justify fixing this. In other words, you may be able to have a welder replace this bracket but you will be constantly 'firefighting' and unable to get ahead of the rust in other structural areas.
In terms of fixing this, the bracket itself is a simple one to replicate, but you'll need to break out a hammer, wire wheel, etc to ensure that the crossmember/tube that it attaches to is solid still, these tubes have fairly thin walls and trap sand/moisture/salt. They can be sleeved from the inside, but again you will need an idea of what this will cost and whether the other areas of the frame are solid enough to justify the repair.
Sorry to hear about the bracket failure and best of luck with the repair, I know that having a rusty frame is no fun sometimes! Almost anything is repairable (ask
@
brillo_76
) but if you'll be paying someone else to do the work there always has to be a line in the sand after which it isn't practical.