Yes, it's a 4.3, and yes I have no problem with that weight. I'm not sure if they put different differential ratios on these or not, but I can pull a lot more with this than other vehicles I've had. I've not experienced the issues you've described with mine.
That's not to say, however, that the '99 Blazer is a great truck....I've had nothing but problems with mine. It's a 3-year lease company vehicle, so next summer new truck time.
Good luck on your decision.