1999. Why doesn't it upshift?

Are there any DTC's - diagnostic trouble codes ?in the PCM / BCM ? You have third an fourth below 50 mph but above 50 it down shifts to 2nd gear ,is this what your saying or you don't have 3rd or 4th gears at all ? Which engine did yours come with ? 2.2 L or 4.3 L You trans can set several DTC'S an not all will turn on the check engine light .
DTC B2722 (4.3 L) DTC P0218 (2.2 L) DTC P0218 (4.3 L) DTC P0502 (2.2 L) DTC P0502 (4.3 L) DTC P0503 (2.2 L) DTC P0503 (4.3 L) DTC P0705 (2.2 L) DTC P0706 (4.3 L) DTC P0711 (2.2 L) DTC P0711 (4.3 L) DTC P0712 (2.2 L) DTC P0712 (4.3 L) DTC P0713 (2.2 L) DTC P0713 (4.3 L) DTC P0719 (2.2 L) DTC P0719 (4.3 L) DTC P0724 (2.2 L) DTC P0724 (4.3 L) DTC P0740 (2.2 L) DTC P0740 (4.3 L) DTC P0741 (4.3 L) DTC P0742 (2.2 L) DTC P0742 (4.3 L) DTC P0748 (2.2 L) DTC P0748 (4.3 L) DTC P0751 (2.2 L) DTC P0751 (4.3 L) DTC P0752 (4.3 L) DTC P0753 (2.2 L) DTC P0753 (4.3 L) DTC P0756 (2.2 L) DTC P0756 (4.3 L) DTC P0757 (4.3 L) DTC P0758 (2.2 L) DTC P0758 (4.3 L) DTC P0785 (2.2 L) DTC P0785 (4.3 L) DTC P1810 (2.2 L) DTC P1810 (4.3 L) DTC P1860 (2.2 L) DTC P1860 (4.3 L) DTC P1870 (2.2 L) DTC P1870 (4.3 L) DTC P1875 (4.3 L) .
Shift Pattern This category contains the following topics:
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One forward gear only

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Two forward gears only

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Gear missing or slipping

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No upshift or slipping upshift

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No downshifts

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Non-First gear start

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Refer to First Gear Range Only - No Upshift .

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Refer to Third Gear Only .

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Refer to Second/Third Gear Only or First/Fourth Gears Only .

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Refer to Slips in First Gear .

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Refer to Slipping or Harsh 1-2 Shift .

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Refer to No 2-3 Shift or 2-3 Shift Slips, Rough or Hunting .

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Refer to No 3-4 Shift, Slips or Rough 3-4 Shift .

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Refer to No Part Throttle or Delayed Downshifts .

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Refer to Second Gear Start .

you locked it into a specific ratio when you went down hill in 2 nd
it will not upshift until you select drive again

Transmission is slipping or not upshifting into 3rd gear ?

I guess my first question would be - are you saying it is slipping in 3rd gear ? If you push the gas pedal to the floor it should downshift into a lower gear.
If it is slipping, or not upshifting into 3rd gear, I'm afraid chances are the transmission is toast.
You can take it to a couple of shops for estimates but they will probably want to pull the pan and take a look inside.

these vehicles have alot of problems with the tcm( tranmission control module) internal failures of trans speed sensors. your car is currently in "limp in mode" to protect your trasmission!!!

its not reading the speedo signal properly, does you speedo wave about sometimes ?

well,whoever did the transmission needs to look at it again.It sounds like a solenoid problem internally in the transmission.or a plug,wiring,or vacuum leak

you have a valve body problem, some of the speed sonsor controls are malfunctioning... need to get into the valve body... check fluid condition for dirty and burned condition...

have you dropped the pan to see whats the bands are set at, any debris in pan bottom, whats your governor pressure, if bands are ok, but gov pressures are not relative to speed 1 psi per mph up to 95-127psi, port is on pass side of case rear top right, scan tool would be very helpful, your must have mil light on for this problem, gov dtc's, replace gov transducer as gov solniod as a pair, no exceptions, also are there any 02 sensor dtc's, shorted to voltage, this wil make pcm do stupid things to trans shift, affecting gov signals. GOOD LUCK RDJRP