Like I'm in neutral

I was driving down the interstate this morning about 55-60 miles an hour all of a sudden the truck went into what seemed like neutral. I pulled over and put it in park. Put into drive, 2,1, reverse nothing, its like its in neutral . The truck is running fine and trying to put in gear does not affect the motor . it will go into park , but when its in drive,reverse 2,1 will just roll like in N. Linkage appears to be in tact. Fluid was just slightly low. After adding fluid, no change, fluid appears to be fine not burnt. No puddles or obvious leak. Towed home sitting in my car port now. I know she has a lot of miles 281000+ but believe me when I say it was smooth riding even up to it happening. I'm hoping for some clues on where to begin, thank you pros.