After LS swap, transmission engagement issues, suspected transfer case problem

Hey guys. I have an 83 CJ7 with an LS 6.3 corvette swap, and an NV4500 5 spd. I just finished my body off resto and swap. I went to do a tune first run and it felt like my transmission was slipping. With a twin stick, left stick all the way to the rear and right stick in the middle should be 2wh. I put my CJ back on my lift and when I shift the left stick all the way forward and the right in the middle, it only engages the rear driveshaft, like its backwards. When i out the left all the way to the rear it engages both driveshafts. So, that is my first issue. My second issue is the slipping feel. If I let out the clutch my Jeep will start to move, but when I give it some gas, it barely moves. Feels like a clutch slip, but isn't. I've taken the transmission back down and out, and checked the clutch measurements and checked the clutch plate for discoloration and slip wear. It still looks brand newish.

After messing with the transmission for a few days, I've come to think this is a TC problem. For one, the shifting is very hard, it doesn't feel like it "settles" into a gear when I do get it to shift and this slip issue. I'm at a loss. I'm not a transfercase expert by any means. Can a transfercase cause a slipping sensation? If so, how do you fix it? It was supposed to be rebuilt.

 

Dana 300? There really isn't any place those can slip. Either in or out in all positions-no clutches of any sort.