Ahhh... good old vibes in the driveline... yuck! Basically a vibe coming for the converter will stop if you tap the brake (this disengages the torque converter), but you can also have other things in the tranny causeing a vibration... Vibes are very tricky... it maybe tires, wheel bearings, or even brakes if it is more speed related... if it happens at the rame rpm, it's probably in the engine... if happens at different rpms it's is usually a driveline vibe.... especially if it is worse on the gas and subsides when you let off. First thing I do is pull the driveshaft and do the u-jt's.... it is the probable cause and pretty cheap to fix... check out the shaft for missing weights, dents, whatever...also make sure the tail shaft and yoke are good and there is no slip... another note is vibes can come from the tcase, but will change when you go into 4wd or 4L. Final stuff is driveshaft balance (unusual) and then the gears in the rear. So there is kinda a diagnois in a nutsheel.
1987 Chevy K5 Blazer- 350 TBI