Welcome to having a young Lab! They are bloody nuts until around 3 and then they start to settle a bit. Your boy is going to need MUCH more exercise than just long walks. Is there some place to take him swimming? That tires dogs out pretty quickly .
With this acre of land, do you just leave him out to play on his own or do you throw a ball for him, use a flirt pole, anything like that? Many dogs don't really have the knack of "self-exercising". If you've got friends or neighbors with high-energy, friendly dogs, a playdate should wear your boy out quite a bit too.
As far as the manners go, that's going to take some work. Labs just have that "HOLY COW HI I LOVE YOU!" thing going and they don't understand that when they jump on you, it's like being hit by a sack of bricks. If he's doing this when people are coming to the door, either put him away for a bit or have him on a leash so you can keep him from jumping. This next part requires cooperation from your visitors, so it's usually a good idea to recruit some friends who owe you favors or something. Also, it's a good idea to get him good and tired beforehand.
When people do come to the door, put your dog into a "sit" while on the leash. Since he's probably not going to hold this at all on his own at first, put your foot on the leash as well as hold it. Don't step on it so far that he's being pulled down to the ground, just enough to keep him from being able to jump up. Have a pocket full of yummy treats as the ready. As someone comes in, your dog is going to go nuts probably. Both of you just need to stand there and ignore him. Don't yell commands at him that you know he's not going to listen to and the other person can't show him a lick of attention either. Just talk to each other until the dog calms down on his own. Then YOU give him a treat first, for being calm and have the other person take a step further in the door. Quite likely, the dog is going to start trying to bounce around again. Have the person stop and resume the ignoring. Wait until Crackers is calm again, treat and the other person comes a step closer.
What you're aiming for is the other person to get within arm's length of the dog and WHEN HE IS CALM AND SITTING, have the other person give the dog a treat or two. But the second the dog starts to wind up again, all attention ceases. He only gets treats, he only gets attention when he's behaving. This will take a while to get through to him, because I'm assuming that up until now, his jumping and crashing has been met with "OFF! OFF YOU BLOODY DOG GET DOWN!" and being shoved away. This is still attention, negative as it might be, which reinforces your dog's behavior.
Hope that's a good place to start! I would also recommend finding an obedience class near you, as hands-on help is always much more effective than reading advice over the internet. We can help you as much as we can, but there are some things, such as timing, that can only be improved by someone watching what's going on.
Don't worry though, this is a very common stage/issue with most Labs. They are a very high-energy breed, especially when young. You have to remember that they were bred to be able to run and swim and retrieve all day long. They do get better though!
