baffled

2006 e-350 passenger van .no power under load from stop on 30 percent grade .
untill i hit 2000 rpm then gets better from there .highway 15 to 20 percent grade runs fine power is fine lots of power at highway speed only issues are there at stop under loed any ideas on what direction to look the motor is a 5.4 just did a total rebuild runs strong im pretty good at turning a wrench .not so good at the electronics of the newer automobiles .not kicking any codes. im stumped could use some direction on what direction to lean twords to find the issue and get it fixed has traction control if that helps thanks in advance for any help:banghead::banghead:

So many ways it could be from experience. Junk in the gas tank that moves to the pickup tube when going up hill. The traction control thinking the strain going up hill is making vehicle slip so it is engaging brakes. Transmission at tilt and fluid low is putting it in wrong gear. Then the various electrical problems that just won't show a code and drive you crazy is just a few of the ideas I've got.

Partially plugged Cat?

thanks for the input im finding the direction im seeking and keep plugging away at it