I now ride a 60cm SurlyLHT with chain rings at 26/36/48t. Can I replace with a smaller set at 22/32/46t (or similar combination) without having to replace the entire drive train - cassette, rear derailleur, front deraileur etc.?
Your biggest concern will be the front derailleur. Any front derailleur will have a maximum and minimum size for each chainring. You should be able to find what those minimums and maximums are in the manual for your derailleur. That said, the change you're making is relatively small and it's not likely that you'll have a problem.
Your second biggest concern will be the chain. The smaller rings will mean that there's more slack in your chain. You may be able to get away with it just fine, but you'll most likely have to remove a link or two for optimal performance. Determining the correct chain length has been covered at length in multiple places on this site, so I won't go into it here.
Going from a bigger front chainring set to a smaller one isn't very likely to cause you problems in the rear. If you were going bigger, it could be a different story since there would be a chance that the derailleur wouldn't be able to take up the extra slack of the longer chain required for bigger rings. But again, not likely going from bigger rings to smaller ones. Especially with the relatively small change you're making.
The simple answer is yes, you can just change the crankset without replacing the entire drive train however there are other considerations. Depends on if your changing rings out or replacing the set and cranks. Other considerations if doing a full replacement are:
When selecting a new crank set you need to ensure it aligns with the current chainline of your bike. If you change the chain line you may not be able to use all of the gears on your rear cassette in all gears up front.
You need to ensure that a new crankset will be compatible with your current BB spindle length and interface. (square, octo, isis, hollowtech etc.) If your BB has a lot of wear once you have the cranks off its not a bad idea to change the BB at the same time as they're not that big a cost. Also it's sometimes if you get a good deal on a crankset to get a matching BB as they're the cheaper item.
You may need to shorten the chain going to smaller rings if the derailleur doesn't provide enough tension.
Rear derailleur capacity
Not a problem when shifting to a smaller crankset but if going larger you would need to ensure that the rear derailleur would handle the additional cogs. There are tools online to help calculate this.
If the chain and rear cassette are showing signs of wear it's often wise to replace these at the same time as they will wear a new crankset faster.
If you are buying a new crankset it has to be appropriate for the number of gears on your bike, this is because chains are different widths to accommodate more gears on the rear cassette. The rest of the drive train is sized to fit the chain width.
You should be able to go to 24/34/46 without having to do much other than maybe remove a link from the chain and maybe slide the front derailer down a smidge.
You won't be able to do 22/34/46 without replacing the cranks, though.
Surly LHT specs: http://surlybikes.com/bikes/long_haul_trucker
Not all LHT model years used the same crank, but they used similar 5-arm 110/74mm bcd cranks in the other years.
Nice chart of the smallest size chainring possible with each 5-arm bcd size: http://sheldonbrown.com/harris/chainrings.html
Sheldon Brown has 33mm as the smallest middle-chainring, not 34, so maybe it's possible to find a 33mm middle chainring... Seems to be rare.
Note that it's also possible to the cogset from 11/34t to a 12/36t cogset to get your gear range lower. I can't find good specs on that rear derailer to be sure the 36t would work,though...
For any gear changes like this, work out what the extremes will be. How many teeth in the small/small gear combo, how many teeth in the big/big gear combo, and how big is that difference?