Shifting AW4 into reverse while going forward

I'd put my money on "it needs to be burped". I know you are aware of this issue, but I had similar difficulty with my '89 after blowing an upper radiator hose. Did repair on the highway with new hose and couldn't, for the life of me, keep it from spewing out the fill bottle. Finally, after about a half dozen attempts, it gulped and started sucking fluid from the fill bottle faster than I could add it! Once on the trail, I had to point the nose downhill so that the fill bottle was the highest point in the system. Once it started erupting, I kept squeezing upper hose...finally burped and then sucked in several quarts. No problem since.