'03 Trailblazer No Reverse cold

On cold days, my '03 4.2L 4X4 Trailblazer won't engage reverse except after a long pause. Dropping the shifter into reverse gets me nothing but another "neutral"; although after ten or fifteen or thirty seconds, it begins to engage especially if I get impatient and start revving the engine. "Slides" into reverse, it's not a normal firm engagement.

Works great in the spring/summer/fall; works great in the winter AFTER the vehicle has run long enough to warm up some. Reverse problems started last winter, went away when the weather warmed up, and came back this winter.

Trans fluid and filter changed recently; no change in reverse function. Old fluid looked reasonably good, more pink than cherry-red, but not burned or evil-smelling. No chunks 'n' pieces in the pan, just normal black/dark grey powder--and not all that much considering the vehicle has 90K on it.

Ideas? Clutch seals? Band servo? Valve body?

Not sure what it is but I have been getting that poblem on my duramax dually.