2005 Mercedes Benz SLK350 Shifer haning

This car was wrecked, and I'm rebuilding it. When I got it running, I shifted it back & forth from R to D a few times without problem, then suddenly it refused to shift beyond R. P and R no problem but refuses to go into N or D. Acts like some kind of lockout to me (like the yellow park lockout key, which doesn't affect this problem), but the dealer tells me I need a new shifter. Help, please!