Holy crap.....I've changed/installed about 1/2 dozen rear u joints in my life...but never had two caps stuck like the ones I'm trying to remove from the front shaft (2000 Silverado 4x4 ext cab) today. There are no locks visible locks on these....I've used the propane torch, big socket, 5lb sledge, vise, nothing gives...not wanting to damage the shaft but crap...guess I'll keep banging the crap out of it unless someone has a better idea...

 

Sometimes those caps are frozen so bad you'll bend the ears on the shaft before you get them loose. I know this may sound silly but I've been able to get low mileage driveshafts complete with ujoints out of a wrecked truck often times cheaper than just 2 new ujoints. You don't have to mess with getting the old ones out and you can just slip the other shaft in. My local junkyard sells shafts with ujoints for only $35.

 

I dont think there are any locks I think you have to heat them up until wax quits coming out of them

 

If they have never been replaced there are plastic retainers that need to be melted. so just heat it up then bang on it.

 

I know its not GM but on the newer Dodge solid front axles they use U joints at the knuckles and they are just an insanely tight fit to begin with plus they rust in. They have to be taken apart with a press and even then its pretty tough. You put all the pressure you dare on it, reach as far as you can and just lightly tap it with a hammer and BAM things go flying and out the cap comes. Sounds like yours is trying to be the same way.