So I don't know hardly anything about transmissions, and I don't know if this is something to be concerned about, but I am curious about it.

So the truck's not my daily driver, and often, it may sit for several days without being driven, but I've noticed that after it sits for a few days, if I go to drive it, I have trouble with the transmission engaging gear, it always engages, but it really takes it's time the first time I start it up. I've also figured out that if I rev the engine before putting the transmission in gear, it engages just fine, no problems. Besides this, it engages all gears perfectly fine without issue, and if I were to start the truck later in the day, it still engages gear great, it's just a little hard after it's been sitting. Any ideas why it does this?

The truck has the 46re transmission.

 

Start it in neutral and let it run for 30 seconds or so and all will be good.
The pump doesn't circulate the fluid good in park.

 

There's a pretty good chance that the anti-drainback check valve was removed at some point, which is a very good thing. When that goofy little valve plugs up it eats the much more expensive transmission.

Though running the engine with the transmission in Neutral for ten or twenty seconds works, it's a bit hazardous. If you forget to do it, or someone who doesn't know to do it drives your truck, things can get ugly. If, say, someone just shifts into Drive and decides to mash down on the accelerator pedal while the torque converter is only partially filled, the vanes in the torque converter will be stressed, perhaps bent, perhaps broken. The stress comes from the vanes being in air above the ATF, then banging into the ATF -- think belly flop. It's also tough on the clutches, which will slip quite a lot until full pressure is developed.

 

The very first and most basic thing to do is check the tranny fluid level. Make sure you go for a drive long enough to get it all up to normal temps everywhere. Have a couple quarts of atf+4 and a funnel that has the small tube on the end. Get to a level place, keep running, set ebrake, and put into Neutral.

Go check the tranny dip stick. If you have to add, then do your darndest to not overfill. 1/4 quart at a time.

Next morning you start, see if any change.