I have the same model, about the same year, very similar situation and I'm having the identical problem described here and by others. (I was surprised to find how many posts 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 later tried again with a spray cleaner, oil, then brake cleaner followed by PB Blaster but still get NO movement.
Searching for info I came across this and thought, before disassembling it, it would be worth a try:
"I would remove the action from the stock then soak the receiver, trigger group and bolt in automatic transmission fluid for a few days. Cut the top off the jug and slide the parts in then rest it against something so it doesn't fall over. 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 article also pointed out something else well worth knowing: WD-40 is NOT recommended for use on guns:
"okay as a cleaner, but make sure that you remove all of it and replace it with a good rust-preventing oil before storing any gun. WD-40 doesn't work very well as a gun lubricant or to prevent rust - it's too light. However, it is useful when cleaning to blow fouling out of cracks and crevices."
Anyway, it's now been soaking for about 3-4 weeks. I pull it out, try to wiggle it back and forth every few days and, though there does seem to be some slight movement now, it's still far from working so, despite the advice some offer to leave disassembling the trigger assembly to a professional gunsmith, I think that's what I'm going to have to do ...unless this is opening a can of worms better left unopened.
Is disassembling the trigger assembly something that can be managed by the average guy with some mechanical aptitude and no specialized tools?
Is it a matter of removing the pins and sliding the parts out?
Are the the small pins just pushed through? Does it matter which direction?
Can anyone please share (or direct me to somewhere that explains) how to do disassemble the trigger assembly.
Also, for any interested that might come across this post in the future, a pretty useful diagram for this model is available
here.
Thanks