Bicycle Mechanics - rear wheel probs

i took my rear wheel to shop yesterday to get some spokes replaced. go back today and they just gave me a new wheel instead. come home put it back together. pedal it on the stand to go thru the gears and now the chain will not even stay on the smallest cog it just jumps back and forth between the last 2 cogs then im watching the cassette and it like floating moving around a little. thats not supposed to happen right? how do i fix this?

First of all it's normal for cassettes to float, so put that out of your mind. Your problem is most likely that the distance from the cassette to the right dropout varies hub to hub. This is something that people who switch back and forth between different wheels are used to. You'll have to adjust the trim of the RD to match the new position. You do this using the adjusting barrel where the housing meets the RD.

Shift the lever to the position corresponding to either the second or third smallest sprocket, (not the smallest). Turn the adjuster while pedaling, until the RD completes the shift to the correct position. Then try shifting up and down, and fine tune until it shifts reliably in both directions.

thank you

You'll also need to check the derailleur's limit screws; they'll likely need a slight tweak.