that's not entirely unheard of you're adding more power that'll make things slip.
you can add a shift kit that will help up the holding pressures on the transmission internals but be aware that the weak spot in a 4R100 is the torque converter and I'm guessing that's what you're feeling slip since it unlocks at every shift
Huh? the TCC doesn't even engage period until the truck is in 4th gear, load is below 18% and road speed is over 45 mph...
But yes, if you're making any kind of power at all, you're liable to be able to overpower the clutch pistons in the transmission. Also, non-"kitted" transmission take forever to apply the next set of hydraulics, so a shift kit will boost line pressure to make it more of a "snap" and hold them tighter.
But yes, I agree... the TCC in a stock convertor is a joke.. hit the "OD-cancel" button if you're hauling anything more than a couple bags of dog food