Erratic steering in 4 wheel drive only

[ColoJeep]

So my grown daughter bought a 99 4Runner , we gave the once over and I found the rear drive shaft was wasted , ( centering ball was toast and the DC joint had a 1/4" run out.
Bought a new shaft and installed it for her , No issues drove great. However on snow packed roads in 4wd the front end would pull one way then the other under power.
In 2wd it drives great no pulling or steering issues at all. Doing a dry steering test the only movement is the steering rack bushings let the rack move 3/16" side to side.
If that is the issue would that only effect the steering in 4wd? I haven't seen anything like this it is extremely darty in 4wd and perfect in 2wd. Thoughts ????

 

[LukesfirstJeep]

Are all tires the same size?

 

[ColoJeep]

Are all tires the same size?

Yes, 225 75 R 16 pressure all at 30 psi. and look pretty evenly worn.

 

[LukesfirstJeep]

The only time I experienced something like this was when I had to install my spare tire on the rear axle of my Tahoe. The spare was slightly smaller than the other three tires. My Tahoe had limited slip, and going from snow packed road to dry pavement it would shoot into the other lane. I put the smaller spare on the front axle, the steering wheel was cocked about 90 degrees to go straight, but I could stay in my lane.

Since your problem only seems to happen with using 4WD, I'm wondering if there's some type of auto locking differential that's intermittent or some other issue inside the differential.

 

[ColoJeep]

The only time I experienced something like this was when I had to install my spare tire on the rear axle of my Tahoe. The spare was slightly smaller than the other three tires. My Tahoe had limited slip, and going from snow packed road to dry pavement it would shoot into the other lane. I put the smaller spare on the front axle, the steering wheel was cocked about 90 degrees to go straight, but I could stay in my lane.

Since your problem only seems to happen with using 4WD, I'm wondering if there's some type of auto locking differential that's intermittent or some other issue inside the differential.

Good thought , however it has a conventional part time T - case ( J shifter in Toyota speak ) and both front and rear diffs are open.
It really wants to go erratic in 4wd. and it's Rolls Royce smooth in 2wd. this is really weird.
I am reaching out because we have all seen weird crap over the years , and hopefully someone has some insight into this mess. Thanks!

 

[LukesfirstJeep]

Pull the front diff cover. Is it possible the "open diff" is binding intermittently when in 4WD.

 

[ColoJeep]

Pull the front diff cover. Is it possible the "open diff" is binding intermittently when in 4WD.

I suppose that's a possibility , but Toyota's don't use a D-30 straight axle. Your not dropping a cover your pulling the whole axle assy. out then splitting the case. Spinning the front driveshaft doesn't feel weird and and both front wheels are powered. Thanks !

 

[ColoJeep]

Solved ! I decided to go ahead a lay in the snow when it's 15F. to Figure this out.
What I discovered is the front axle is a 4.1 ratio ( what the 4Runner was built with ) , But someone installed a 4.3 ratio rear end.
After running the numbers about a million times , I found I could stagger the tire sizes enough that it drives fine in 4wd.
Running a LT225/75R - 16 on the front and a 265/70R - 16 on the rear gives me enough stagger to equalize the ratios !
Doing the math it is within 6 rpm at 60 mph.
This is good news for by broke daughter and son in law. It drives perfect in 4wd now.
I guess watching years of roundie round racing payed off.

 

[sideproject]

Solved ! I decided to go ahead a lay in the snow when it's 15F. to Figure this out.
What I discovered is the front axle is a 4.1 ratio ( what the 4Runner was built with ) , But someone installed a 4.3 ratio rear end.
After running the numbers about a million times , I found I could stagger the tire sizes enough that it drives fine in 4wd.
Running a LT225/75R - 16 on the front and a 265/70R - 16 on the rear gives me enough stagger to equalize the ratios !
Doing the math it is within 6 rpm at 60 mph.
This is good news for by broke daughter and son in law. It drives perfect in 4wd now.
I guess watching years of roundie round racing payed off.

View attachment 395680

I'm Impressed Nice job

 

[LukesfirstJeep]

Good on you for figuring it out.