If it were a sensor, it would throw a "P0700-Generic Transmission code", in which a dealer would have to investigate.
Shift solenoid engages reverse.
I've gotta say that I've never ever seen anyone have trouble with the 545RFE transmission. In stating that, they're pretty cheap to replace, as there's TONS of them from wrecked vehicles (it was used in about 10 different vehicles), so they're everywhere.
After a bit of googling (again, never heard of a problem with this trans) I found this:http://en.allexperts.com/q/Dodge-Rep...ny-reverse.htm
It applies to Rams, but it's the same transmission. Since you picked it up cheap, it may be worthwhile to have the dealer look at the software on the Transmission controller.