Okay, so you are running the old V6 "cologne engine" used also for the for explorer made from "solid steel" all the way. Very nice picture by the way your rear suspension, compliments!!. Yes the engine is indeed very heavy and given the fact that you cannot change that, I would assume that you should run rather stiff front springs to get a decent ride frequency out of the car. This should in theory permit you to run a softer rollbar favouring less understeer. I assume that you did put the roll center back-up for camber recovery but why not try to do this with more caster ? A McPherson strut, even with a rather high rollcenter has very little camber gain recovery so this could be "easily" achieved in a different way. You need camber in cornering and increased caster (have a look at the WRC Focus world rallye car with a very rearwards inclined strut) will give you that. That would make a raise of front roll-center not necessary and cause less trouble at the back end of the car. You could also modify the front suspension to have a more significant amount of anti-dive by lowering the front A-arm joint/ raising the rear A-arm Joint which also will increase caster on the outside wheel during cornering. The very nice thing about caster is that it will make the car roll "more" without activating the front rollbar (just think of a go-cart wheel jacking with steering) but it WILL load the rear rollbar up, so it is a double whammy for reducing understeer. Beyond that I also fear that your AWD setup might be biassed itself to much towards "understeer". In general every AWD tends to inherently understeer more, so if there is a way for you to split your torques front to rear diverse this might help you more than anything else (running any LSDs ?). OK lets recap .. you also said that you are suffering from traction at the rear. So ... you have understeer and not enough traction at the rear ... actions .....
1) better traction rear can be provided by less roll bar rear ... ok, going for this, lets assume this resolves the traction issue
2) less rear rollbar causes more understeer ... should i go up with rear rollcenter ? To some extend yes but carefull about jacking, raising rear rideheight in cornering will not make my car much faster .. probably.
3) ok ... I cannot go up too much on the rear rch then. Can i go down on the front ?
4) sure, yes I could but I probably loose a lot of camber gain on the front ...hang on this is a McPherson strut, they do not have a lot of camber gain anyway .... could this be an idea ?
5) sure, but how can I fix my camber gain ? I could also increase static negative camber ?
6) Static negative camber will help but I could also use increased caster to get more negative camber angle gain with steering

Food for thought ...
Cheers, I am going to bed
dynatune,