I have a 89 K1500 w/700R4 and have not had this problem, but this is what I found on the net.

also try searching on www.fullsizechevy.com

Hard 1-2 Shift
Concern:Some 4L60E applications may exhibit a hard 1-2 shift condition.

Cause: This concern may be caused by any of the following conditions:

# #8 check ball may not be sealing properly on the spacer plate or it may be missing. This check ball has a tendency to severely peen the spacer plate.
# Several DTCs may result in increased line pressure. These include slippage; ration error and several of the solenoid DTCs.
# Broken, damaged, or incorrectly installed 1-2 accumulator spring/piston.
# Sticking, damaged, or incorrectly installed accumulator valve or spring.
# Drill and tap 2-4 servo cover and note the pressure at the cover while making a 1-2 shift. If pressure is present, inspect the 4-3 sequence, 3-4 relay valves. Clean and repair the valves as required.
# Calibration was released to address this concern on some 4L60E applications.
# 4-3 sequence, 3-4 relay valve train sticking.

The most common problem is the spacer plate being damaged by the #8 check ball. Replace or repair the damaged spacer plate.