4x4 vacuum modulator????

if I excellerate fast enough to drop down a gear <passing gear and so on > as soon as I try to stop.<stop sign ,red light ,ect> I seem to have no vacuum. Brakes are hard and truck wants to die,resembling trying to stop a stick shift without using the clutch,but it does not completely die if I keep the r.p.m.s up.I get the jeep stopped and shut it off.a few seconds later I hear a pop under the hood.Poof all is fixed,till it does it again.someone at murry's says it could be a vacuum modulator for the 4 wheel drive.does anyone else possibly know what this might be?

What year jeep? The "pop" sound you hear could be from the timing being advanced too far. As far as your vacuum problem, double check all your hoses, especially the one to the brake booster. Actually, if it runns good during normal driving, then I would look into maybe a new booster, the "pop" could be the booster releasing inside after the vac. pressure is releived.

it is an 88 with 4.0

Yes, the problem IS in your brake booster. The booster is designed to hold vacuum,(using a one-way valve) enough for one or two pedal applications with the engine turned off. This is a government required safety feature and has been present in all vacuum boosters since the 1960's.
In your case, this one-way valve is not working. The booster only has vacuum when it is being generalted by the intake manifold. When you open the throttle, the intake manifold vacuum goes to zero. In your case, the vacuum in the booster is lost so you get hard brakes.
Just change the brake booster and you will be fine.

I replaced the check valve going into the brake booster.it seems to have worked but I have thought I had it fixed before.If nothing else my brakes seem more responsive.

died on me again this morning,only this time no more "pop"