4 speed box feels like extra gear?

I have the 4 speed Auto gearbox and just wanted to check is 5th gear an overdrive so on the 5 speed box it would have a 6th gear as overdrive? If I go from 2nd to drive (which I assume then puts me to third?) then it will sit on about 3000rpm at just under 100k. and does not shift from there. When I turn on overdrive then it will drop down to 2200 rpm and take me to about 105/110 which I assume is 4th, and then if I back off the throttle slightly it shudders and seems to drop another gear, the rpm drops and its like it has entered a 5th gear? Running 33's BDG A/T. I read that the shudder might just be old/bad oil? have bought all new full synthetic oil to change it hope it helps.

The 4 speed auto's have a lock up torque converter so what feels like 5 the gear is the torque converter locking up to give better fuel economy on the highway.

What Sherro said... but, as you mention... it definitely shouldn't shudder.
When the lock up solenoid ports pressure to lock the Torque Converter it should be hardly noticeable except on the tacho.
I would have a tranny flush and full inspection done not just change the oil but up to you.

Ok thanks for the info, I am putting new filters and oil in the transmission today got the full service kit for it so hopefully this will make a difference

How many kms on your car? My 03 patrol had the same issue (shuddering) which a flush resolved (or masked) for a couple of months but I had to bite the bullet and get the torque convertor replaced.

From memory the shudder is the gearbox struggling to get the right line pressure as the lining of the torque convertor is causing blockages.

Paul