2001 Silverado hard shift lever when cold

My 2001 Silverado with 5.3 engine and auto transmission has a problem when it is cold - like around zero degrees like it was this morning here in upstate NY. The shift lever is very hard to move into any gear. Once the engine and transmission warms up, it shifts normally. Once it is moved into gear, the transmission works normally.



The transmission has been serviced every 50,000 miles (GM recommends every 100,000) with Mobil 1 synthetic transmission fluid and a new filter. Trans fluid is at normal operating level. Any ideas would be appreciated as to shy this is happening. Thanks

I should have added to the above post that the truck has 170,000 miles on it and has only occasionally pulled a boat trailer or carried a load - nothing too heavy.