NP203 problems with 4H on part time kit

[dkstanley]

Hi all, new to the forum. I've been putting a 78 3/4ton K20 back together and have an unusual problem i can't seem to find anywhere on this forum. I have an NP203 transfer case with the part time kit, TH400 transmission, and a SBC 350. I know it has the part time kit because the hubs are locking and when i put it in 2wd Low i can turn the front drive shaft with the hubs unlocked and the same when i put it in 2H (used to be 4H without the part time kit). My problem is, in 2wd Low the truck does what its supposed to do, drives down the road with the hubs unlocked and locked, all be it at higher RPM's due to low gear, it also works well in 4wd Low, but when i put it in 2wd High, it bogs the motor down and dies when put under a load. Any ideas what could be causing this?