Thanks ReAX - go figure - about 5 minutes after the post I found the answer I was looking for. Turns out the stocker is 3 Ohms, so 4 ohms should be close enough. I'm not going for anything major so I'll just drive a fairly efficient 8" driver from the stock amplifier. I'm also not really in the mood to run all new power & gnd wires, that's why I'll keep stock
Looking at the JBL GTO series since they seem to be inexpensive and higher efficiency (89-90dB) Kickers are looking good too. I'll see what they have on sale at my car audio place up the street.
Starting to come back around. At least I've got a lot of oxycodone. I've got 2 screws and a bone staple in there now. Having the left foot done in October.