Truck takes a long warm up to go into drive (or reverse) gear

It sounds to me as though the fluid seals inside the transmission are not sealing properly when cold. This can be due to old/dry seals. I would suggest you add some additive to the transmission fluid made to soften up those seals. Other than that and/or if the additive does not work or only works for a while, I am afraid to tell you the transmission will have to be rebuilt with new seals and might as well completely rebuilt it then.

Good afternoon my name is Roy and I will assist.

From your description it sounds like the filter is clogged in the transmission.

I would start by dropping the pan, inspect for any debris in the bottom magnet and changing the filter. See if there is any improvement.

Try this and report back to us with the results.

Roy