2000 Durango, 5.9L, 2wd, 85k on the odo. The wife is having intermittent issues with the tranny not shifting into fourth gear - sometimes it will, sometimes it won't. I took out the speed sensor located at the rear of the tranny to clean the tip of it off (sometimes metal "powder" accumulates on the magnetized tip). Hopefully this will work. Any other suggestions? Other possible causes? No CEL/check engine light coming on either.

Could be the trans output sensor located on the drivers side middle of the trans. Very easy to replace.

I would replace both of them together.

Then get the trans fully serviced. Here is some info I put into this already. possible transmission problems

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The lip seal for the overdrive piston could be leaking, causing the overdrive problem. You can replace just the overdrive piston seals with out pulling the transmission out. You would need to pull the overdrive unit out.

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Master Chassis tech, Transmission
Automatic and Manual and Suspensions.
Also repair front and rear diffs and all engine
repairs inc Diesel. A/C repair too.

I have a similar problem, except my '99 Durango 5.2L will shift into 4th (overdrive). But driving at normal highway speeds on flat roads, it will sometimes shift back up into 3rd and wants to stay there, causing an increase of 800-900 RPM. It reports an error code of P0783, which is listed as a 3 - 4 shift malfunction ("overdrive solenoid is unable to engage the gear change from 3rd to the overdrive gear.") Is this absolutely the overdrive solenoid, or could some of the other sensors be at fault? (which are much cheaper to replace.) The transmission has not been serviced for about 40,000 miles, but the fluid is not discolored and does not smell burned. I am reluctant to have it serviced if I am going to have the transmission taken apart anyway, but it might be a good gamble if servicing might correct the intermittent problem. Fluid level is correct. Any advice?