they replaced my transmission

Took my son's truck in and they inspected it and said it needed a new transmission. They put a rebuilt in less then 6 months ago and now 800 miles later the transmission is broke again. Took it back in and they are telling me it will cost me 900 dollars to pay for labor. They said the u joints are bad and that it's why they are not replacing it for free as the warranty they gave me stated. They are ripping me off, they were paid to inspect and repair the truck. The u joints should have been inspected when they initially inspected the truck and replaced if they were worn. Plus, I have looked at my warranty work and the put down the incorrect year and odometer reading on the warranty,WHY? I am taking the truck to be inspected at another garage to verify I am being ripped off.