It might be a combination of everything. the engine isn't making quite as much power, the tires are slightly bigger, the clutch is a bit worn.
The reason I ask about the tires, I stepped up to 30s in my 4.0 '88 (back when it was 2wd) and noticed the same thing you're experiencing, just from the tire size increase. when the rear axle blew up, I swapped from 3.07s to 3.55s and it all went away and i was happy again. :D