I'm not sure exactly what happens but if my truck sits for the weekend on monday morning it seems like it goes in gear but wont move for 15-30 seconds almost like it has to build line pressure or something. I have noticed this regardless of temp and revving engine slightly does nothing but you can tell it definetly is in gear.

Almost sounds like convertor drainback issue... wasn't there a TSB on it for 2001-3 trucks?

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I suggest you refer to posts in 'Converter drain back on an 05' and 'Trans Shudder & Not Engaging when Cold' threads as there is some very good info about the converter drain back issue there.