My 4.0 V8 has always accelerated pretty poorly. Even on a slight hill it'll drop down to 2nd to accelerate from 30 to 40mph. Even at about 50mph it won't really accelerate on a hill. Now here's the thing, everyone says the cams wear really quickly but every description I have found of a worn cam is that it'll accelerate just fine at low speed but run out of puff as it gets up the rev range, and that makes sense.
Mine goes like a bloody rocket above about 3000rpm. Give it a hefty bootful and once it's moving it just pulls and pulls, especially on LPG. It does sound a bit rattly at about 4500-5000rpm in "Sports" mode but it's as quiet and smooth as you like below that - except there's an irregular "slappy" rattling from somewhere down below. I thought it was to the back of the engine but I'm coming round to the idea it's at the front.
Could it be that the cam is actually okay, and that the chain itself is excessively worn and stretched? I don't really know the engine's history, since it was bought second-hand as a replacement, but it seems clean enough. I put a set of second-hand rocker shafts on because the originals were badly worn which makes me wonder about the cam, but when we had it apart to fit it, everything looked pretty much okay. Eyeballing the rockers as the engine turns over they all seem to move about the same amount. A leakdown test showed about 20% evenly across all eight cylinders, which isn't great but isn't particularly horrible either - certainly not enough to make me want to pull cylinder heads off!