I lift my gmc sierra and i got bigger wheels. It became so heavey! Can't move?

I have a 3500HD sierra 2008 6.0 vortec, i lift the car 8inchs and got bigger tires, it became so heavy, like seriously i can't use the 6th gear on 60mph, i have to keep it on 5th or less. Its so slow, make sense the tires are heavy, but its not that big its still 17 size of the rims, i've seen people in california has dodge, chevy, fords with alot bigger wheels and lifted more than 20inch, and they drive it daily. I'm i missjng something? Couldit be the diff ratio size? Trans program? Cause the truck is stock at the monent, just lifted and bigger tires

Its not just me, most my friends here have american trucks and lifted and have the same problem, workshops here are useless. Any tips?

You have geared up the vehicle by fitting larger tires. So 6th gear is now more like 7th, and you don't have the torque to drive in the new gear. Acceleration in each gear is also slower, as you are effectively starting off in 2nd, and then every gear after that is also taller. As 6th gear is an overdrive anyway, gearing that taller makes it useless.

To make it work you need a lower ratio diff to return the gearing to normal.

Yes the wheels and tires are heavier, so the vehicle will be slightly slower, but that's not the big issue.

Also don't discount that you actually have mechanical issues with the engine. A 6.0 l engine should still push a small stuck along once you get up to speed.