In reverse the sensors do not play a part although it is possible that there is malfunction with the shift switch and the ECM is mistakenly actuating one or both of the shift solenoids when it should not. Quick way to test is to disconnect the shift solenoid connector (round white connector going to the ear right of the trans) and see if the problem still persists. The hard shifting sounds like the problem is internal - could be a sticking shift valve (both 1-2 and 2-3 shift valves are used in R) or a sticking piston in the direct clutch or the first reserve brake. Sticking valves are not too hard to work on as the valve body can be removed from below but servicing the clutches will require the removal and disassembly of the trans.