2006 dodge 3500 4x4 tranny issue
 

when I first start my truck and shift it in gear the tranny does not engage immediately, it takes 10-15 seconds to engage. All the fluid levels are good and once I drive it this doesn't happen , it only happens when after the truck has set over night or for a day or two. I pull a 12,000 pound horse trailer and it doesn't slip but i have notice it not wanting to shift into 4th gear without letting my foot off the throttle.