I Just SASd a toyota t100 with a landcruiser front and stock rear and changed the gears to 529ʻs front and back and now the truck rocks back and forth repeatedly when driving in 4x with the hubs locked. I cut the perches off turned the axle up to fix the drive shaft the shortend the drive shaft so it has in and out play but the problem still remains.. IDK but it seems like the prob is in the front third member. Also it runs good in 2 wheel.

 

What do you mean "rocks back and forth"?

 

OK I played with it again today and am pretty positive its in the transfercase. I think since its automatic the gearing in the T case doesnt match the 529ʻs. When I put it in 4 low and watch it slow the tranfercase kinda jumps up and down but not real dramatic. I run it in 2W and it was good, then lock the hubs remake another front drive shaft because I thought the single yoke was failing and put in a double yoke and still the same. jacked the whole truck off the ground un bolt the rear drive shaft, thinking because it was a little off balance, ran it and still the same.. Also as I let the truck roll in 4L I feel the front axle and theres no rock or unusual movement in that, but hte transfercase moves alot.. I want to know what gears I would need to get for my trans OR if i could go w/ a different ring and pinion gear to match the automatic trans.
and to answer your question I think from the trans jumping that is making the whole truck rock.

 

Sounds like you are running 2 different ratios in the diffs, like 4.88 in 1 and 5.29 in the other.

You don't match the ring and pinion gear ratios to the t-case or the transmission gear ratios, you just match the front r&p to the rear r&p. Since it doesn't have problems in 2wd, that means to me that the front and rear axles aren't meshing with eachother and your t-case is paying the price. Don't use 4wd anymore, there might still be a chance your t-case hasn't already grenaded.

 

Any updates OP? Just wandering...

 

Hey Brian yeah still havent figured it out yet it.. I jacked the whole truck off the ground all tire NOT touching. Put it in 2wd slow, No thump. Hubs locked In 4L Thump , in 4H fast thump. Then I disconnect the rear drive shaft so no connection to the back R&P and still a thump. And while its thumping I feel the front axle and drive shaft and theres no irregular thump or hop or anything but the transfercase is where its at its bounces up and down (if you go fast) and you can see it.. Also I remade 2 different drive shaft to eliminate that route and STILL..

 

Possibly a bad T case???

 

Sounds like it ... disconnect the front drive shaft and see if it still does it in 4WD