Steven Moore's The Novel: An Alternative History, 1600-1800 covers this topic in depth. It traces the novel's history and talks about pioneer books like Tristram Shandy and how they fit into the art form's development. It will also make your reading list much longer.

You might be a bit overstating the surrealness of the novel. The reason it seems modern is partly due to it's lewdness and partly to it's perceived meta-ness. We forget the Victorians did a number on censorship and what was considered acceptable to discuss and read and we are in some ways still dealing with that fall out. Prior to that, a lot of popular fiction (of which Tristram is a part) dealt with bodily fluids, sex jokes and fornicating clergy. (If you liked Sterne's dirty jokes, read Gargantua and Pantagruel)

As for it's plot and voice, the novel frequently breaks the fourth wall and talks to the reader in ways you wouldn't expect, like the page with the plot arcs, Uncle Toby's walking stick gesture or the Black Page. There were really no rules for the novel when Sterne was writing this work and he would not have been thinking about it in terms of a novel as we understand it. The story was originally published in installments and was not bound as one full work. Given it's letter like form it's not at all strange that Sterne talks to the reader or plays visual games with them, however, just because it feels like meta or post-modernist, doesn't really make it so. Post-modernism, for example, comes with nuclear annihilation baggage, anxiety of new capitalism and a restlessness expressed through transit as well as the non-linear plots it shares with Tristram. This isn't to put down the novel in any way, (it's one of my favorites too) but it would be better to categorize it as innovative rather than post-modern.

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No, I don't think you're overstating this. I have found this to be an interesting question: the relationship between early literature and post-modernism.

First of all, we need to clear up what you mean by post-modernist--it is, after all, a very poorly defined literary "movement." If you're referring to John Barth, Donald Barthelme, William H. Gass, Italo Calvino, etc;--One could also argue, Thomas Pynchon and David Foster Wallace should be included--then I think there is a marked difference between Tristam Shandy and these authors. I will expound upon this if this does align with what you meant when you used the term post-modernist.

I should also point out that Tristam Shandy is probably not the only work that one could make a similar argument with. I think one could make a better argument with Rabelais or Cervantes. In both Gargantua and Pantagruel and Don Quixote, the author uses post-modernist (and modernist) ideas throughout the work. And these two works pre-date Sterne. It seems to me that this type of work can be traced all the way to The Romance of the Rose (even though that work is very different from what we're talking about here,--it's a sustained allegory written in early poetic form--for its time, it was a unique work and influenced many of the great authors of history) which was published in about the 13th Century. Then, after that, I would be remiss if I didn't mention Erasmus' In Praise of Folly and Thomas More's Utopia, both of which were written in about the late 15th, early 16th Centuries.

That being said, and as I've already alluded to, I think there is a marked difference between the post-modernist I mentioned (Barth and Barthelme especially) and this early literature. Are you familiar with these post-modernist writers? Is that what you meant when you used to term?