You can run stock speakers off a hu no problem. Too high of ohms will never hurt it, too low will. Speakers are always at different ohm loads anyway, what the manufacture says is a low. A 4 ohm speaker could be a 200 ohm speaker at a certian frequency.
I dunno whut they did, but the stock speakers in my 03 are pretty nice.... I swapped the head unit out in the first week I had the truck and was waiting for the paper speakers to fall apart. To my surprise, they sound better now and haven't heard any complaints from them(cracking or popping and the like...) And I'm fairly pleased with the quality of sound as well.... I know it doesn't have much to do with the conversation, just thought I'd share!
Just replaced my stock speakers in the 97 extended cab today and the doors were 4 ohm and the rear 4x10's were 10 ohm.
Sounds hella better.
Am planning on making 2 boxes tomorrow for my 8 in Kicker subs that are going under the jump seats. I don't like to advertise as it gets gone in a hurry round here.