700r4 problems

[66gmcguy]

I have a 1983 Blazer with a 6.2 diesel and 700r4. When taking off from a stop, the transmission starts out in 2nd instead of 1st. It will only start in 1st gear if I manually shift it into 1st. It seems to only do this after the trans warms up a little bit. Otherwise, everything else seems to work normally. The shifts feel good, no slipping, and the fluid is clean and full (checked when running and warm.) It downshifts if I step on the accelerator when climbing a grade or trying to pass someone. The problem is only when coming up to a stop and trying to take off.

Any thoughts what this might be?