I had this same problem on my truck this past fall.

This is what worked for me.
Sprayed with penetrating oil. I like PB Blaster. Then smack each screw with a hand held impact driver like the one jnichols suggests. A BFH works well with this tool.

Then get a #3 phillips bit for a ratchet/breaker bar. Loosen each screw a bit and apply more PB. If you get the scews moving and they start to get harder to turn, drive them back in a bit, apply more PB and wait a day. This got five of the six out for me. The last required drilling. Drill carefully and get the last bit out with the right sized tap so you don't ruin the captured nut inside the jamb.