Dear Larry,
You must/have to/ have got to realize that the examples I give are where there is a difference in meaning.
The reason that they are often interchangeable is that the underlying difference, external versus internal obligation, is by no means clear.
"You're really going to have to pull your socks up"
"You really must pull your socks up"
Any difference in meaning; I doubt it.
should, must, etc.
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Surrealia also asked for an explanation of the meaning and use of "ought" and I guess someone should respond.
I see a link between ought and owe. Ought means "to owe it to someone to do something" i.e. to have a justifiable but non-enforceable obligation to do something
"I really ought to stop smoking" = "I really owe it to myself to stop smoking (for health reasons)" or "I really owe it to the guy sitting next to me to stop smoking (because he is allergic to smoke)" etc.
"The government ought to reduce personal taxation" = "The government owes it to us, the people who put it there, to reduce taxation (because it was promised)" or "the government owe it to themselves to reduce taxation (because they are losing popularity)"
Harzer
I see a link between ought and owe. Ought means "to owe it to someone to do something" i.e. to have a justifiable but non-enforceable obligation to do something
"I really ought to stop smoking" = "I really owe it to myself to stop smoking (for health reasons)" or "I really owe it to the guy sitting next to me to stop smoking (because he is allergic to smoke)" etc.
"The government ought to reduce personal taxation" = "The government owes it to us, the people who put it there, to reduce taxation (because it was promised)" or "the government owe it to themselves to reduce taxation (because they are losing popularity)"
Harzer
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LarryLatham
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I agree, Harzer. 
I sense, also, that ought to is, in many cases at least, close in meaning to should. There is a sense of obligation there. They aren't the same, however, because one of them is a modal auxiliary while the other is not, even if it does suggest some degree of modality. As we all know, modality is perhaps the messiest area of grammar in English. 
Ah...Stephen...
The second speaker above, on the other hand, is personally insisting on it, as, for example, like a mother would to a young son. But I'll not insist that because it's clear to me it ought to be clear for everyone else. 
I will assert, however, that native speakers generally do, even if often without being conscious of it, make a distinction. If they truly were identical, the result would be mass confusion, because a speaker has to choose between them somehow, for a decision to use one of them is also a simultaneous decision not to use the other, and if there were no difference, how would she make the choice? A mental coin-flip, perhaps.
Larry Latham
I sense, also, that ought to is, in many cases at least, close in meaning to should. There is a sense of obligation there. They aren't the same, however, because one of them is a modal auxiliary while the other is not, even if it does suggest some degree of modality. As we all know, modality is perhaps the messiest area of grammar in English. Ah...Stephen...
...I don't know...perhaps it is difficult for many people, even native speakers, to articulate a difference in meaning here. It is clear for me, however, that the speaker in the first sentence is suggesting some outside reason (such as, perhaps, because it's really cold outside and your pants are too short to cover your ankles; or, maybe because you're on your way to school, and the teacher likes neat socks) why it appears necessary to pull your socks up. Context may make the reason clear. Or not. Listeners can only surmise their interlocutor's reasons for saying what they say. Note that it's quite common, in conversation, for someone to say, "Uh...why do you say that?" Or, "Beg pardon?""You're really going to have to pull your socks up"
"You really must pull your socks up"
Any difference in meaning; I doubt it.
The second speaker above, on the other hand, is personally insisting on it, as, for example, like a mother would to a young son. But I'll not insist that because it's clear to me it ought to be clear for everyone else.
I will assert, however, that native speakers generally do, even if often without being conscious of it, make a distinction. If they truly were identical, the result would be mass confusion, because a speaker has to choose between them somehow, for a decision to use one of them is also a simultaneous decision not to use the other, and if there were no difference, how would she make the choice? A mental coin-flip, perhaps. Larry Latham