Miss in engine after hill descent

[mudrat]

HI TRITON BUFFS
I was wondering if any one has had the same problem & has found a solution or ideas for my problem?
If I go down a steep hill in low gear low range, when I get to the bottom of the hill the engine chugs as if it is only running on 3 cylininders. It eventually clears, but sometimes it takes a while (sometimes 20 minutes). I tried changing fuel filters, full tank of fuel, same result.
The baffling thing is that it does not do it in high range or two wheel drive on the same hill.
I was thinking that in low range it may put more pressure on the motor which may make one of the injectors leak diesel into the cylinder which is not burnt during descent.
Mitsubishi changed my manifold recently (on a recall) & I changed the muffler for a straight through muffler (hotdog).Could this cause the problem wich may cause reduced backpressure on the motor?
2008 GLXR Dual Cab (Olympic Edition)
3.2 Turbo Diesel Manual
101,000 KLM