My 98 Deville is very hard to shift into gear when it is cold. It works fine when it is warm outside,only in cold weather can I hardly move the shifter. Anyone know what is the cause so I can have it fixed? It feels like something is restricting the movement of the shifter, like trying to go past something rubber. Anyone?

 

Possibly the shifter cable itself is binding. When really cold, the outer sheathing can contract, and cause the inner cable to drag/bind somewhat.
Best way to determine if it is the cable, or something internal on the trans is to disconnect the linkage on the trans(while cold), and see if the shifter is still hard to move. If it is, you should replace the cable. If it moves freely while disconnected from the trans, there may be issues inside the trans causing it.
If your car has the externally mounted neutral safety switch/manual lever position switch(nss/mlps), those can also bind up when cold, causing harder than "normal" shifter movement. The MLPS will be mounted directly underneath the shift cable linkage on the transmission, if it is external. Can't recall off-hand what year they went to an internal mlps, but I think it was around 2000-2001.