My mechanic is stumped...
 

2006 durango. 4x4. 4.7 v8. 150,000 miles. Auto.

On flat ground runs fine. Going uphill revs way up with very little if any speed increase. If i keep the revs down there is no power to accelerate and I actually start slowing down. Sounds like a classic cat problem but I replaced cats about a year ago. Mechanic says the aftermarket cats "might" be bad and we should replace them with new ones. It's a walker which from what I understand is a good aftermarket brand. I've told him to hold on replacing for now.

No check engine light and no codes.

Has anyone else had this problem or have any other ideas as to what could be wrong?

my guess, fuel pump

It could be a timing chain jumped a tooth but generally, the 4.7 will hammer the valves if it jumps time. Before I replaced the cat., I'd hook a vacuum gauge to the engine and see what it pulls. With the recent warming of the weather, you might have rodents in the exhaust stopping your gasses from getting out rather that a bad cat.

I don't know but if the revs are coming up normally without acceleration point more to a torque converter/tranny issue. If the engine/revs spinning up where is the power going... Nowhere it seems. Just my two cents but I world eliminate that issue unless I misunderstood.