In the winter, cold weather will affect these struts (old struts lose Nitrogen over time and extreme cold exhibits this sign).and need to get replaced. Shop around as they are expensive. Dealer get $ $80 or more a piece for these. Yu should change both or they won't work well together.
Your call. Get them aftermarket if yu can. $129 pair? depends on vehicle year and model.
Shop around.
Generally I feel they are overpriced wherever yu get them as they don't last very long considering their price. But, check with the dealer first and ask for their BEST price, then use as reference to shop further.
Easy to do yourself.
replace one at a time. Have a good helper to keep the door propped up while changing so yu don't get bopped in the head or worse...get a concussuion. It is straight forward.
I usually try to get them on O/E warranty just before it is up. They end up failing on ALL cars. nature of the design. Usuall one goes before the other but becomes more noticeable as both start to wear out. Then all of a sudden the door falls with out notice.
Good luck
LT