1991 Astro van vibration upon acceleration

1991 Chev Astro CL AWD EXT. 163,000 miles on a TBI 4.3L. Purchased in 2000 with 110,000 miles. The $6500 van turned into a $13,000 van. Have completed transfer case upgrade - rebuilt trans and front diff. Replaced drive shafts and U-joints, and a broken trans mount. Replaced all steering rods and ball joints. This took care of most vibration, but recently have experienced vibration shudder upon acceleration, and especially when climbing hills. It is a drone noise that can be heard, and felt under the front buckets when driving. Highway is smooth sailing when on level surfaces. Could it be the internal engine balance that you mentioned in another member's Q? Might I use a different transfer case fluid that would help?