heavy vibration at 50-55 mph?

[79 D100]

when im going about 50 mph my truck starts vibrating quite violently. When i let off the gas the vibrations stop. Its a 79 D100 with 318 from a '68 barracuda and a 3 speed w/ overdrive tranny from an 82 D100. Could the the drive shaft be vibrating or maybe 4th gear is messed up? it doesnt seem to happen in 3rd gear. Any help appreciated.

[69Dodgedart360]

a countershaft bushing is likely worn or your countershaft could be...bent? im not sure how possible that is though

[6 packin]

There is no 4th gear actually in the trans. It should just be when the main shafts locks togather for the final ratio. If the noise is only in 4th gear, its a bearing. Most likely the front main shaft one or the back main shaft.

[DODGEZILLA]

Check your motor mounts, you may have a cracked one. Also your u-joints & driveshaft. Did this come on suddenly or has it been doing it for a while? Has it progressively gotten worse?

[79 D100]

it just started recently, i looked in the haynes book and it says it could be a driveshaft problem. im suspecting the universal joint because it also says that if there is a knock or clunk when shifting gears it could be the u joint, which i also have a problem with.