1997 2500HD 46re 4x4 3.55 ratio. It seems i have 5 gears, 1 2 3 4 overdrive. If i use the O/D button, shifts 1 2 3 4, if while in 4th, and engage O/D, it feels like it goes back to 3rd, then 4th, then another RPM drop into overdrive. Are those tach jumps and falls from the transmission shifting or the TC locking/unlocking?

 

1 to 3 then overdrive. It's my understanding that lock up of the converter happens after the OD engages. The lock up should be only about 200 rpm's. If it's more it may be dropping down a gear. Do you have any codes? Possible the pressure sensor/solenoid are going.