95.5 Rodeo transmission problem

Newbie, here, so please bear with me. I'm no mechanic, but fortunately am married to my mechanic's sister! Seem to have a transmission problem with 95.5 Rodeo. At stoplight, took foot off break and when pressed accelerator to go forward, heard a snap or knock somewhere about the center of the car. Started a major shimmy at low speed and idle, and then again at about 50. Got to a spot to pull over, nothing appears snapped, brakes ok, trans. fluid fine. This is the first problem we've ever had with the transmission. Drove -- shimmying and shaking -- s-l-o-w-l-y home, shifting into 2nd on our lake road. Felt immediately like driving in 4L. Shifted into 1st to take the steep grade up to my house, amazed I topped the hill. Couldn't get it above 8 mph. Of course it happens Friday at 4:45 - will have to worry and wonder all weekend.

How bad is it?