99 Century tranny/differential problem

I have a 99 Century, Engine puked at 80k, replaced with 97 Monte 3.1 with 99k a year ago. Driving down the road heard a clunk and maybe something drop? Lost all gears, forward and reverse. Neither wheel spins in park so I do not believe it is a broken axle. Fluid level is fine. No loud noises when put into gear, shifter lever on tranny moves, but car does not. My dad thinks it could be in differential. Any suggestions would be great, thanks.

does your fuild level drop from where is it before you start the car when you start the car? I'm thinking the pump shaft or input shaft broke. what color is the fluid. when the diff goes it usally puts a hole in the extention housing on the passenger side.