Ford E350 with 5.8 stalling sometimes when shifting into reverse
 

Hi,
i got everything together today, changed the transmission oil and filter, filled it with approx 12 quarts of oil now it appears full on the dipstick, however It sound a bit strange in reverse, has a errrrrrrr sound when i'm backing up, also it stalled the engine a few times right after shifting into reverse, but other times its backing up just find, but has that noise, it seems like its no longer slipping, think it's my torque converter low on oil from some kind of reason? i also notice it's lacking power going down the road. how hard is it to replace the TC? what other seals would need replaced aside from the torque converter? please let me know, thanks,

thanks for the help