the 01 V6 extended cab is a flange pinion mount with a double cardan rear section. thats a real needle in a haystack, and most used units will have the same problem (the double cardan is worn out). I sold just the rear section of one for $150 from a low mileage truck a couple years ago and could have probably asked twice as much with how much my phone rang. you cant easily convert to a one piece because of the crossmember and the proximity of the driveshaft to the floor. plus the length of a one piece can cause problems (harmonics). some guys have converted to the 2wd 4 cylinder ext cab driveshaft which doesnt have the double cardan, but I think you need to change the pinion too and I really have no experience with that swap.
midwest driveshaft
http://www.mdsdriveshaft.com/gmc.html sells a replacement with or without the double cardan but its pricey. I have had good luck with driveshaft work at inland truck parts on west k42, their driveshaft guys are great, they call my s10 stuff "toothpicks". dont go in the main door, there is a "driveline shop" door to the left.