May have a boost or fuel problem?

Ok fellas, I just recently switched into 2001 Pete 379exhd 18speed, c15 475/550 single turbo. Truck pulls fine thru 1-17 but soon as I put her in 18th she acts as though she falls flat on her face on the smallest hills. I find myself down shifting to get the rpms up and then trying again. When I do shift to 18 the rpms kinda hang between 1000 and 1500 then once on level ground she runs like a scalded dog. Definitely a head scratcher, anybody know anything feel free to help. Thanks in advance.