My friend had this happen on his 91 K-5 blazer. The gears were all in the proper spot, but the indicator didn't line up. It was something like the length of a cable being off.
My first suggestion would be make sure you can get to all your gears. Put the shifter all the way up into park, then count the "clicks" back, P...R...N...D... However many numbers there are on the shifter. If you can get into the proper number of gears, then its ok unless your ocdish and need them to line up, you'll get used to it.
If you cant get into all of them I would take it back to the garage and tell them "I can no longer get the shifter into this position and the gear I am in does not correspond to what is shown on the shifter."
Something along those lines.