jdm93400

hey guys i have had this not so bad thumping sound that happens mostly when i first drive it. like it sounds like a bad motor mount but had all them checked out. but not bad. i just got a used transmisson put in a year ago.
it shifts strong. im thinkn maybe the diff has something wrong or the drive shaft

so my differnetal i dont think iv ever changed the fluid how bad is that . iv had it only for 2yrs

my tranny fluid is a dark red how bad is that i just had it changed with new filter a year ago. dont know how many miles i put on it

any help would be appreciated