Long shot but. on my '98 I had a similar problem. Long story short, it turned out the speed sensor wire had been partly pulled out of the connector at the PCM. After a new PCM and speed sensor and hours of searching, pushing the wire back in place and taping it securely fixed the problem.

 

Thank you...going to check for codes, disconnect (+) and reset apps.

Update: crank position sensor (cps) code came up. $67, replaced for the second time in 5yrs. Hopefully this solves the shift issue.

 

Wellllll....clearly after CEL and code present, the CPS is not the issue. Guess I'll take it to the same tranny shop that ran the diag last time. PITA

 

No trans codes, no CEL. Going slow truck will eventually slide into 2/3rd gear, once there shifts to OD and all the way down easily. Down shifts on hwy great and pulls hard...just a problem off the line. Trans shop says time to drop it as it appears to be internal problem =estimate $2300.

 

After tear down, the trans shop stated this was a factory trans (not sure if its the orig, 235k mi) as 3rd gear seal was the original factory culprit. Seal fails and won't hold fluid allowing trans to stay/slip in 3rd gear. Full overhaul, rear main crank seal replacement and doing the pass axle seal also.

 

These transmissions use solenoids rather than shift servos to select the appropriate circuits to engage the right band/clutch combo to make each gear.

These solenoids get sticky, weak, plugged, or more commonly "tired" which makes them sluggish to engage, especially when hot.

You can get the whole solenoid set for $150 off e-bay (oem parts). This issue swindles many a truck owner into buying a new transmission. It's wise to buy the solenoids as a set.

Perhaps not related to issues in thread, but something to be aware of.