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The first thing to do would be the check the transmission fluid level. If it's low enough it will slip until the transmission fluid warms up a little.
If the fluid is normal, then there could be internal wear in the transmission that might require a rebuild, or in some cases a band adjustment. Is your truck*****
There isn't a dipstick, but there is still a method for checking the level. Here is the diagram that shows how. It's unlikely that it would be low without some kind of fluid leak, but then again I have seen them from the factory with low fluid (albeit this one was in the factory 100k miles ago).
It's just the first place to check. If everything is normal, then there must be internal transmission wear causing the slip, most likely the C1 clutch pack (see attached TSB).