Something must be wrong! I have a 02LS! with 4L60E, 3.73 gears, 30.6" tires and locking diff and I can spin both tires all day long. My truck feels strongest in low gear and pulls real hard through all the gears and I'm running the stock converter. Have you tried to shift to low gear and take off vs just leaving it in drive. Maybe it isn't shifting into 1st and it is taking off in 2nd. Count your gears as it is shifting and see if all four gears are there. When I first drove mine I forgot to hook up one power wire and my truck felt real slugish and took off from 2nd gear unless I manually shifted it. Just some thoughts.
2006 Superior Blue Trailblazer SS AWD, Stock as a rock
Sold: 2002 GMC Sportside Denali front end with a 2002 LS1, FLT level 5, Yank 2600, Trick Turbo, T76, Nelson intercooler, 60LB injectors, Warbro fuel pump, Eaton locker, Magnaflow, 3.42, Nelson Performance Tuning (speed density).
Sold: 1981 GMC LB RC 1500 2002 4.8L 4L60E 12 bolt 3.73 Richmond Lock Right AC PS Nelson Performance tuning