I have a 22re 4x4 with about 400k miles on the transmission. I get a vibration in 4th and 5th fear when I let off the throttle. Or when going down hill at part throttle. It only happens when I let off the throttle or re apply the throttle.

My transfter case output yoke home has some play. It doesn’t leak but I’ve tightened the nut (which was loose) and it still has play and the truck still vibrates. The driveshaft and rear end seem tight. The rear engine mount seems ok.

am I looking at a transfer case problem? Bearing issue? Worn out output shaft?

 

Could be multiple things.

Drain the oil on the diff, that's a good start. U-joints might appear to be good, but when have they ever been replaced? How's the transmission mount? What about the rear drums?

 

Seems like a smoking gun from here. As the load changes on the output shaft your drive shaft will change angles slightly which is hard on the ujoints and the loose shaft bearing. If the drive shaft flange angles aren't aligned the shaft speed will oscillate in a sort of surging fashion. (Whomp, whomp..)

Checking your diff fluid isn't a bad idea, when mine was still full of muddy water it sounded was more of an audio que (jet engine going reverse thrust sort of howl) than a vibration.

 

It is possible to replace the transfer case rear output bearing without complete disassembly or removal of the t. case from the truck.

If there is substantial play there after the flange is torqued to 90-95 FP, the bearing is worn.