To do intramuscular in the rear leg, I usually put the animal on its bottom and inject into the fleshy thigh facing me. At the front of the leg, not the back, provided there is enough muscle there. (Sometimes you're injecting lambs who aren't doing well, and there's precious little muscle anywhere.)
Our vet did recommend using the muscle at the back of the upper front leg (pull the leg forward and inject into the muscle that connects the shoulder to the rib cage) but on hill and primitive sheep I found there wasn't as much muscle there as I'd like.
Then I was shown how to inject into the muscle in the pelvis, going through the gap formed by the spine and the pelvic girdle, and I've used that technique ever since. I much prefer it to the other sites. But get your vet to show you how to find the right spot.
I've never done I/m in the neck, didn't know you could! Always a school day here on TAS