Yes, you generally want the pinion angle to be 1-2 degrees below that of the transfer case output flange to allow for uptilt under load.

Although since you are having trouble at no load/coasting, that may not be the problem.

You may also have just put the overall driveshaft angle beyond it's limit with u-joints. I found on my '85 that happened when the shaft hit someplace in the 12-15 degree slope range (about 4" lift and shortened shaft with dual cases). Nothing would make it run smooth until I had a CV joint put on the top end and rolled the pinion angle up to align with the shaft.