One of the reasons the 2000 performs so well with that motor and drivetrain, is weight. The 2000 models weigh significantly less than the 86 model. Even the 86 model weighs less than a 76 model. GM tried to drop weight where ever they could. Items like the aluminum NP 208 transfer case compared to a cast iron NP 203 or NP 205, dropping from a GM 12 bolt rear axle to a GM 10 bolt rear axle, it all adds up to less weight. Also the GM 14 bolt 9.5" ring gear axle, as opposed to the 10.5" ring gear 14 bolt axle. GM did it to satisfy the EPA on emissions ratings, and MPG ratings.
One key factor in builing up your 86 that will keep it easier, is that it seems to be 2wd. You didn't say, but you only mentioned swapping in the rear axle, so I made the assumption.
Mike: The rear need not be changed, but a gear swap may be in order. I have seen many trucks with strong motors, but they never got the power to the ground, due to a high gear ratio (numerically lower).
A truck with 4.11:1 gears will hop off the line, compared to one that has 2.56:1 gears. At the same time, the 4.11 equipped truck will be turning much higher RPM's on the highway than the 2.56 equipped truck. Having the OD trans helps correct this though.