driveshaft knock

I have replaced my guibo and center bearing about a month ago. Say I engage the clutch at 1000rpm and then go half throttle or more to accelerate, the driveshaft starts hitting the floot. I took a look
at the center bearing, it has some cracks on it, but not torn apart. Seems like the center of the driveshaft is resting on the bearing, because it is squashed on the bottom.
Maybe I didn't put the driveshaft back right, maybe I overtightened the nut that connects the two halves together, what's going on here? It only
happens in first gear around 1500rpm. If I floor it after 2000rpm in first or any rpm in any other gear, it doesn't knock.
Please help.

You have a driveshaft U-joint siezing up. They are not replacable unless you are a machine shop!! The whole driveshaft needs to be replaced. I recommend a rebuilt one with replacable U-joint(for further inexpensive repair. Very common problem on all '80's and early '90's bmw's
They run $600-$700 dollars.