91 chevy 4.3 engine

I have this engine in a 86 CJ-7 jeep. It has 2500 miles on a complete rebuild. Bored .030 crank turned .010. I recently started getting a vibration at 2500 rpm. I get this on the road or on stands. I have removed the accessory belt, disconnected the torque converter and no help. I pulled the engine and tore it down on the advice of a engine rebuilding shop and found no problem. I replaced the harmonic balancer and seemed to help for a while but the problem is back. It starts at 2500 and smoothes down at around 3000. There is no smog junk on the engine. I'm running a 4 dl carb and dual exhaust. All help appreciated.