Did you replace the water pump? There is a big problem with getting the wrong pump - one turns one way with the V belts, the other turns the other way with Serp belts. Parts houses and rebuilders get them mixed up. The difference is the impeller.
I believe BOTH the 4.0 and the 4 banger have the same problem.
Symptoms - very low, if any, flow. Overheats very quickly.