1993 Jimmy No trans codes, bad shift

I have searched all over this site for this problem.

Riddle me this:

1993 GMC Jimmy S-15
No codes stored - just code 12 NORMAL

Transmission works fine in 1-2 shift, plenty of power shifts ok
At about 30 mph or so, torque drops out (slips), engine revs, won't shift into 3RD.

If you manually put it in 2nd, no slipping at all.

But weird thing is, No code. I work with automation equipment so I am familar with computer control. Sometimes a computer won't show a code when it sees nothing it expects.

VSS seems ok.

Only other prob is the ABS light is on most of the time. I thought maybe it was the ABS switch at the brake pedal but I tried disconnecting the wires to see. Same thing. Then I shorted the wires at the disconnected plug. Same thing. So I don't thin the ABS is causing it.

I know someone out there has got something on this.
Thanks

Replace your tranny fluid and install a new transmission filter.

Already did. Did not help. What else?

I would still have the computer scanned, there could be codes stored.

The computer was scanned after the fulid/filter change(it was test driven after the change). Still no codes. ABS has codes, but the Engine and tranny have none.

Thanks for the reply.
Next Step?

Anyone else have the same issue?