th 350 problems

i recently put a rebuilt tranny th350, and from day 1 it leaked out of the the little breather hole in the slip yoke. it never leaked on the old tranny (also was a th350). i took it back and they said it wasnt leaking. i assumed thas it was just overfill getting past and leaking out. but then the snow started melting and i noticed lots of transmission fluid all around my yard, i would have to top it up every week. I took it back again and was told that it was a seal from my tailshaft. they said it was good to go now. but it leaked again. could there be somthing wrong there? it seems the more i drive it the more fluid comes out. so it went back and they welded it closed. when the truck is at idle it seems fine runs great but as soon as you put it in gear it seems like there is no acceleration and like it just flattens out, and sounds rough(not the same as it was before). when i pull my drive shaft a lot of fluid comes out.