growling and popping front diff

hey guys i have a 97 mountaineer v8 awd. Last year my front diff started growling very loud sounded like a bad bearing. I checked through all the standard stuff cv joints, wheel bearings, tires was all good. I got a used front diff just over 20,000 miles on it and installed it noise was gone. Well its back again but now when i hit the gas hard it pops like gears jumping or somthing. I removed front driveshaft took it for a spin no popping but the deafining growl is still there. I guess my question is what is causing this could the transfercase be an issue or did i get a junk diff? Any suggestions would be great. I really dont want to put another diff in till i know what is causing this. Thanks

i bet if u jack the front off the ground and spin the tires u will find which spindle brg is bad

Thanks for reply. I just had it in the air everything is smooth spinning by hand im positive its not wheel bearings. Growl doesnt start till about 10 mph. im thinking its a bearing in the diff I just dont know why it happend again so fast.
Thanks

jacl it up, reinstall yr driveshaft, put it on jackstands, put it in gear an let it run get down there, u will hear every squeaky brg, every iffy u joint, i bet u will find it when u cab hear without road/wind noisee, i have traced down many errant clunks, squeaks this way,, dont run it fast this way, not needed and it can hop off jackstansd if the tire are on and spinning fast,, buck

Thanks ive been reading and jacking it up seems to be a popular dionostic tool i'll give it a shot tomorro. Another thing when im driving at hwy speed and go over a hill it stops untill weight settles then its back.

Well I found the popping. Its coming from the t-case chain or d-shaft splines as far as i can tell. Now im wondering if the t-case caused the diff to go bad the first time. I have not put it up on jacks yet its cold out here. I pulled it on a steep hill in drive let off gas and it didn't roll back. I've read this could be the viscous coupling. geeez more money. Anybody have this happen would like to here about it. Thanks

it sounds to me like the CV joint on the front driveshaft is gone. it is an extremly common problem with the AWD trucks.

what type of noise does a bad front driveshaft make? Is the sound similar to having "Super Swamper" tires rolling on the road?...that off-road tire sound?