how does a transistor behave when it's broken?
In my experience, power transistors fail shorted.  All the pins will measure low resistance to each other.

TIP122 is in TO-220 case.  All three leads are in a row, at least in the linistepper.  The way to desolder this part is to add solder until all three joints are bridged in a big solder blob.  Melt the blob, then pull the part out.  Then clean the solder blob up with a solder sucker, then braid.  Do not attempt to desolder one pin at a time.