My understanding was that you can't treat swayback in a lamb, you have to give copper as a supplement to the ewes mid-pregnancy to avoid getting swayback in the lambs. I would be very happy to be corrected, if there is a treatment for affected lambs.
There are some ailments which cause stiffness, lethargy, etc, in young lambs, some of which seem to respond to antibiotics, some of which may have been transmitted by ticks. If you have ticks about and your vet says you have the tick-borne sheep diseases in your area, then an anti-tick treatment may be indicated to protect the others in your flock - but check the withdrawal period, especially if treating older lambs who are within a couple of months of finished.