2003 ford explorer drive train

My 2003 ford explorer, V6, full time FWD seems to be sliping in first gear. It only happens after it has had time to heat up and when I excelerate aggressively. The transmission is sealed so there is no way to check the fluid level. I am wondering if this is the transmission or some other component of the drive train? I just had the rear end rebuilt and wonder if the differential in the front end may need the same. Any one had a simmilar problem or advise will be helpful.

I believe those transmission can be opened and the fluid replaced without too much trouble. However someone who knows more about them will be along soon. How many miles on this transmission? I would guess by now I would have found a way to change it likely twice.

If Transman sees this thread and replies, his word is golden on transmissions.

84,000 miles on the transmission. When i took it to the dealership for the rearend repair i asked for a flush and fill of the transmission. They said that a flush would clean residue off of the clutch plates and make the transmission fail. I asked if i should do anything and they said leave whats in there in there. I thought this was kind of odd. maybe i should just have the tranmission fluid drained and refilled. I cant see any harm in that. any thoughts?