1989 K1500 transmission shifting strangely

I have a 1989 K1500, 4x4, ext. cab, 350 TBI, auto OD tranny. At 50 to 55 mph, flat road with no load, it feels like the transmission is trying to shift between 3rd and 4th. If I accelerate, the problem goes away untill a no load condition is achieved. If I put the tranny into third, the problem goes away.
The PO had the tranny rebuilt by a reputable firm apx a year ago. The PO also stated he personally put a new torque converter in less than a year ago
What could cause this issue?