my 78 chevy 4x4 it's 1/2ton with 12 bolt rear 6inch lift,35 muders. the truck is shaking in 3rd gear / i swapted the turbo 350 & put in 465 manual with a 208 tcase had to legthen the drive line just got back today, it was balance. did not have the front drive line on it's in the shop to. could that be it maybe, the front stabilizer shock is blown if that helps any. any info would help she's got lot of bugs that iam working out thx pc5150  

the front driveline missing shouldn't cuase any vibration but when i did tranny swap in my 78 blazer and had to modify shafts, i drove it for a week and there was plenty of vibration then had it balanced and now its perfect so it make me wounder the trueness of you balance? also hows your pinion angle that could also cause shakes and even are you tires balanced?  

is the "shakin" in the steering wheel or do ya feel in the seat of yer pants? Generally, if it's only the steerin wheel with a shimmy-shake, it's a problem in the steering(front tires, suspension, or steering)...if it's in yer seat look at rear tires first, then drive line, etc  

Is it just third gear or is it speed related, ie same speed revved out in second?