New carrier bearing mount already broken on driveshaft?

[72farmer]

I have a 67 SWB with a 700R4.It has been lowered about 4" by PO by cutting front springs and a rear shock and spring kit.In Oct. the center carrier bearing housing broke where it mounts to the truck crossmember.I took it out and took it to a driveshaft shop they installed me a new carrier bearing and checked everything else.
I installed it while it was on a 2 post lift with suspension hanging.Now bearing mount has broke again after only a 100 miles or so.
The only thing I can think of is maybe I should have tightened up the 2 bolts on the carrier bearing at ride height instead of on a lift with suspension hanging.Could the driveshaft moved enough in ride height to put pressure on the bearing mount?The crossmember on truck that holds the carrier bearing is slotted.What else could have caused the bearing mount to break?