There are several things that can cause you situation.One of the first things you should do (besides checking the fluid w/engine running -warmed up,which you indicate you have already done) is to scan the powertrain control module with a class 2 scan tool.
I have seen more of the reaction sun shell come apart than anything.
Input Housing Assembly (602)
Manual Valve Link (89)
Valve Body Assembly (60)
Reverse Input Clutch Assembly (605-614)
Lo and Reverse Clutch (694-696)
Reaction Sun Shell (670)
Broken spline on reaction sun shell/replace shell