So it is. Would have been good for the poster to update on the issue. Looks like he has not been back since the post.

Hi guy's, I've recently brought a 06 5.7 limited navigator to replace our JK, love the comfort and power but it doesn't feel like it is selecting OD, the auto shifts fine but when it's in drive you can move the shift over to 4 and there is no downshift, has anyone heard of this, I've been searching but haven't found any mention of it.

Never heard of that. Rule out of the obvious first
I assume it's showing "D" on the dash?
It'd do what you describe if you accidentally had it in "tow/haul", but would show "4" on the dash.

What speed & load are you doing?
Obviously too slow and you might not be in OD to start with, and too fast it wont shift down to 4.

I guess you've got a good feel or ear for autos?
With the MDS coming on/off and the TC locking/unlocking, I have trouble judging the shifts sometimes.

When it's showing D on dash and you shift to 4 nothing changes, doing around 100 last night with light throttle I thought it would have been in OD but I haven't driven one of these before, what speed/load would OD normally be selected.

Hmm yeah that doesn't sound right. I don't know accurate speeds/load off the top of my head, but I can move between D (OD) and 4 at around 100.
I _think_ I'm at about ~3,000rpm in 4th at 110 with 32" tyres, I'll let you do the math for smaller tyres haha. It's fast enough that I notice "hmm, seem to be revving a hit high" if I forget I'm in 4th.

So if the rpm feels comfortable to you (around 2,000 I think off the top of my head.) you're probably in OD.
Maybe something is up with the semi-auto shifter.
Try putting it in tow/haul (button on the centre dash, near the C/T.), that holds it in 4th, and should show "4" on the dash.

Do you think it's activating MDS (shutting down 4 cylinders) at cruise?
It should do it at any speed over about 40 when warm and at minimal load (like <20%.), you can spot it by a small jump in RPM (~200-300 I think.) and the exhaust note changing to a lower pitch drone.
MDS relies on correct oil pressure & flow rate, and has a lot of sensors and crazy logic circuits related to it, I wouldn't be surprised if those circuits can interrupt shifting between OD & 4th for some reason.

If you're not sure what oil it has now, might be worth putting a 20wt in (I use Mobil 1 0W-20) just to rule it out.

Thanks for the help, I'll be going out soon so will check rpm at 100, I plan on doing a full service in the next few weeks and will be changing all fluids, not sure re mds and I have used the tow feature and the revs do increase when that is pushed.

No worries. Almost certainly dropping to 4th in that case (when tow/haul is on.), so OD probably is working. Does the dash show "4" when tow/haul is on?

Maybe something with the shifter. Does it work correctly in lower gears?