code 24 on 88 blazer 4x4 tranny swap

[Joeglock40]

I just bought a 88 blazer 4x4 k5with a 350 tbi engine. The previous owner installed a new350 and swapped the manual tranny for a 700r4 with a 208 behind it. It runs great but only on the highway it pops the CEL on n off every few minutes. The speedometer is working but I know it's a code 24 the vss. The truck also has a b&m type ratchet shifter. Everything seems to be adjusted right but it won't kick into4th gear until I'm at about 60 mph I'm guessing it had to do with the code 24. So since the Speedo is hooked up n seems to be working, what should I do?? The truck runs n idles good. But it's my first tbi Chevy so this obd1 is new to me. Any ideas???
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