BillBren 
2008 Toyota Avalon Sedan XL 4dr Sedan (3.5L 6cyl 6A)
Check the tranny fluid
 Regarding my 2008 Avalon - On 4 occasions I have put the shift lever into reverse and it doesn't engage right away. On one occasion it shuttered when I backed the car up. The car doesn't seem engage in reverse and then slips. I had a very experienced mechanic check the tranny fluid. You have to drop a small pan in the 08 to check the fluid and it had small metal shavings in the pan. I reported this to Toyota USA.