GM-GMC: 2001 S10 Driveline Vibration, double cardan, ujoint

Question
Hello,Did see that you do not answer trans. questions, but I am not sure that is the problem. When starting out under load there seems to be some vibration or thumping at about 10 mph. I put the truck on stands and with no load ran it in drive ("truck is automatic")and at 20 mph can feel the vibration, seems to be at differential. Could it be a Ujoint? Is there other checks to isolate the problem. Thanks Again!

Answer
That is a text book description of a u joint caused vibration.  Remove the driveshaft and check the joints individually.  I will warn you, if you have a double cardan style joint in the centre they are very tricky to service.  Make sure you set the park brake and block the wheels before you crawl under and remove the driveshaft.
Hope this helps.