2000 Ford Ranger XLT Automatic trans. 4X4 3.0L

read the SM,. service manual
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https://eautorepair.net/Marketing/Default.asp

for whAT

1. EGR
2. EVAP
3. BRAKE BOOSTER
4. PCV
5. IAC
6. FPR
7. MAP

ALL? THEN RTM

U-joints would be one thing I would inspect. Check drive-shaft slip yoke for play, ring and pinion for slop, Also trans mount, engine mounts and rear leaf spring and shackle bushings.

check your kick down cable adjustment and fluid level

There is a transmission ID tag located on the trans with the type engraved on it.

Study up on what an Idle Air Control Valve does then make your decision. They can be expensive.
Search fault symptoms

Check your rear end to see if you have a sensor like in this video, if so it could solve some issues. The video describes items all going out around the same time. Its based on a 2 wheel drive but who knows you might have the same sensor in the same location. This is a shot in the dark, but who knows it might help or point you in the right direction.

How do I change the 2003 Ford Ranger XLT 3.0 L Flex Fuel -Auto Trans 2wd Camshaft synchronizer and sensor . Do I need any special tools ? Are there after market parts that are precise replacements for the OEM parts ?

Sounds like the transmission fluid is getting too viscus. Might want to check it, or replace it. I would just get the trans fluid replaced. Costs 80 dollars or can dIY for 25-30$

This video shows the speed sensor and its location for a 2000 ford ranger xlt. It also describes the problems a bad sensor can give.

Sounds like the shift kit is warn out in the transmission. The overdrive line may not be able to provide enough hydrolic pressure to engage the gears to move the truck. When you switch to regualr drive, that line is able to build the hydrolic pressure to engage the gears. At a certian speed (for the transmission, certian hydrolic pressure) there is enough pressure to engage the gears in overdrive, but go below that, and you have to start in drive again. If this is the case, there is no easy fix for that and I recomend taking it to a transmission shop to have your shift kit examined. Opening up an automatic transmission requires specalized tools, clean shop enviroment, and knowlage more then the average back yard machanic has.