88 bronco with a 302 transmission problem

i have an 88 full size bronco 4x4 with an automatic transmission. after the truck is driven for about a mile the tranmission starts to slip and within a few seconds i loose all gears. when i stop and try to shift into park, i can hear the gears grinding like they are free spinning. the truck will not engage any gears unless it sits for a few days. when this first happened it developed a leak that seems to be coming from the point where the fill tube connects to the transmission. i have dropped the pan and the fluid was clean with no metal flakes or peices in the bottom of the pan. i changed the filter, gasket and fluid and its stil happening. any ideas? thanks