My '96 Tahoe (2 door, 4WD, LT) has 136k miles. Master cylinder is type code JB5. Power brake assist (booster) is provided by vacuum on this vehicle (some have hydraulic assist). There are no visible brake fluid leaks and the reservoir is at the proper level. The last brake work done was when I put on new pads and cross drilled rotors on the front and new, US-made drums on the rear about two years and 6,000 miles ago (don't drive it much). There have been no braking problems or symptoms of issues until it just suddenly started with markedly increased brake pedal travel and brake warning lights in the instrument panel remaining on continuously. Both the RED color exclamation point ! and the P symbol (next to one another just to the left of the ABS light) are on continuously. The ABS light is not on after the usual start-up self test sequence. The vehicle does stop with longer than normal pedal travel and a perceptible shudder just prior to coming to a full stop.
With no visible leaks I am thinking I have a bad brake booster and/or internally leaking master cylinder.
Any recommendations appreciated.

Did you check the wheel cylinders for leaks?

Or the rears out of adjustment or something gone kittywhompus?

Like the last post said, It defineatly sounds like a rear wheel cylinder or brake line.
Many times people think that automatic adjusters will adjust the rear brakes. Unfortuneatly they need to be manually adjusted first. You need to tighten them up until the wheel can not be turned. this insures that all the components are set correctly, then you back them off until you feel very little or no drag.
If the brakes are not properly adjusted, the wheel cylinder cups will travel beyond their normal range - into the rusty area. You may get away with resetting the normal range, however I would suggest changing them.
As usual, lube all the moving parts with never sieze or suitable lubricant, backing plate pads, adjuster asm, emergiency cable etc.