Went to check the differential fluid today. The fill plug, threaded, 3/8 drive is frozen,
which appears to be a common problem with the Colorado and Canyon pickups.
Your repair manual may be misleading, as it claims there is a drain bolt/plug.
After some research, I found that many are like mine, there is no drain plug.

IF you intend to change your gear oil, rather than just topping it off, you must
remove the rear pan. In that event, be sure the fill plug is loose first, or you may
wind up refilling your diff thru the vent hose, a complicated procedure.

B'dee, b'dee, b'dee...

 

Can't you pull the cover and then replace it with an aftermarket cover that has a plug?

 

Yes you can. You can also use a universal drain plug kit.
The OP is an FYI for those who think all Colorado differentials have a drain plug,
as most of the service manuals state, which is obviously not true,
and to allow for extra time to loosen the frozen fill plug.

 

I'm down south, so my drain plug ain't frozen