Same rifle, same model, age, etc. and having the identical problem described here and elsewhere by others. (I was surprised to find how many post and discussions there are about this same problem with this same model.)
I took it apart and oiled with no change, later took apart again and tried PB Blaster into all available nooks,crannies and holes in the trigger assembly but, still, the trigger remains frozen. I tried again with a spray cleaner followed by PB Blaster but still get no movement.
I found some that suggested this:
"I would remove the action from the stock then soak the receiver, trigger group and bolt in automatic transmission fluid for a few days...Sometimes this can free up parts that have been frozen by evaporated oil. After that try some non chlorinated brake cleaner to clear out any sludge that might be left. Make sure you really oil it after the brake cleaner as the parts will rust very quickly if left unprotected."
(Looking into whether automatic transmission fluid was really safe to use, would effect bluing etc. I was surprised to learn that apparently automatic transmission fluid is considered superior to (and less expensive than) good quality gun oils. This was new to me, as it may be to some of you. One of the articles also pointed out something else well worth knowing: WD-40 is NOT recommended for use on guns.)
Anyway, it's now been soaking for about a week. There seems to be some slight movement but it's still not right so, despite other's advice to leave disassembling the trigger assembly to a professional gunsmith, I guess I'm about ready to give it a try -- unless this is opening a can of worms that can't be managed by the average guy with some mechanical aptitude. It sounds like you've already done this. Please advise. I couldn't see a way to disassemble the trigger assembly and still successfully get it back together again. The only "fasteners" I see are the small pins. Please tell me/us how it's done.