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Most likely the cam follower of the lift arm has gotten stuck due to some fly rust.
try putting it in draft control and push the top link bracket rearward into a tree or pole, (you need to overcome the spring force) and hopefully that will release it without having to take the 3pt cover off.
Maybe it already pops loose from moving the D-M-P lever alone.
Then, what colour is your backend oil ? creamy white from moisture absorption ?