You may have to adjust the throw on the shift cable. At the end of the cable where it attaches to the shift lever on the tranny, there is a black sliding piece of plastic covering a white button.
Put the truck in park and chock the wheels. Get under there and find the end of the cable. Pry off the end of the cable from the ball on the end of the shift lever. Slide the black retainer over and push out the white button. The end of the cable is spring loaded. This will allow more or less travel on the cable... it essentially shortens or lenghens the cable by a small amout. Adjust it so the end reaches and can snap back on to the end of the lever. Now check to see if you can shift it into first.
If you still have issues you may need a new cable or worse the column shifter mechanism is getting porked up. Here is a link to my experience with this.
http://tahoeyukonforum.com/forum/showthread.php?t=49530