'88 suburban

hoping for some help, feel kinda smoked as i am a mobile heavy equipment mechanic. here it is, 1988 gmc suburban 4x4, 350, auto, 4" suspension lift, 35" tires, quad shocks & dual str. stabilizer.
wicked vibration through front of truck @ certain rpm's, only under load(slight accelerating), only @ exact rpm & or speed. slow up one mph, or slightly touch gas pedal, vibrations gone!! never did this until a few months ago, always lifted w/ these tires, always rode great.
i have replaced motor mounts, tranny mount & u-joints in rear drive shaft. replaced rotors, calipers, pads, checked & regreased wheel bearings. replaced drag link & both ends. checked the entire front end, everything is tight. had tires balanced & rotated. shocks not new, but highly doubt they could cause this. from reading various posts, i am leaning toward the trans. (700r4) causing this, nothing i have done has even touched on this problem. not wasting $$, truck needed those parts, just really starting to lose sanity @ tracking this prob. down. any help to look in the right dir. greatly appreciated. john