I've got a 4L60E in my Tahoe acting up. Here are the specifics.

I rebuilt it 2 years ago with a master rebuild kit that included valvebody upgrades, etc., due to a leak in the cooler that allowed water in.

No codes.
Reverse always works.
I must manually shift into 1st and rev the motor for the trans. to "hook up".
Almost like I'm running a stall converter.

After this initial "hook up", I can shift to Drive and it shifts normally.

I've heard that it may be the forward sprag, but would like any convincing evidence, etc. before going through this thing again.

Maybe the filter came loose causing low fuild pressure I would at least drop the pan and check.