1990 ford ranger shifting

when I go into a new gear on my 1990 ford ranger short bed, it seems to like thump, or like push, its not anything too big to me, but I'm afraid its alot worse, the transmission was rebuilt last year, I also think the transmission leaks fliuids. when I put alot of fluid in, it doesn't seem to do the thump/push. if anyone has any ideas please let me know, thank you brandon