Please help diagnose a problem
 

Back in November I brought my 98 4Runner into Toyota of Bourne Texas to have several issues worked on. One was a front axel with busted seals. They told me they needed to replace the front axel. Ever since then my 4WD system has not functioned properly. Yesterday afternoon I took my truck out for a quick drive with the 4WD engaged to keep the system maintained as says in the owners manual. Yesterday I drove 16 miles with the four wheel drive engaged, driving 30 mph, up to 50 mph a few short times.

During the drive yesterday I could hear a high pitch wine. The faster I would go the higher the pitch of the sound. Toward the end of the drive smoke was coming out from underneath my truck. I immediately disengaged the four wheel drive. However the 4WD stayed engaged for 5 second. When it finally disengaged the truck jerked violently forward. As if someone hit me hard from behind. I then immediately smelled an odor of rotten eggs or sulfur. It was definitely a rotten burning smell. I pulled the truck over and raised the hood. Smoke was coming from underneath the engine.

Please share your thoughts on what the issue may be. Thanks.

my hope would be that you didn't fry your transfercase. When you smell a sulfur smell you're smelling the additives in the gear oil that is found inside your front and rear differential and your transfercase. Were you off road when you got your truck up to 50mph? Or at the very best driving in an exactly straight line? I would diagnose the issue off of the road on a dirt road. See if all your 4x4 works as it should still. If it is, maybe consider checking the levels of fluid in your transfercase and differentials.You could have possibly even had a leaky seal and little to no fluid in the tcase, so when you went up to 50mph the components inside started running dry, hence the smoke.

EDIT: What do you mean when you say your 4x4 isnt working properly? In the next sentence you said you went ahead and engaged it, what about the 4x4 system isnt working properly?

If the front diff was leaking, there's a really good chance you ran it low on lubrication. That, combined with running the 4wd on a dry surface, would set up a perfect environment for the diff to heat up to dangerously high temps.

The smoke and sulfur smell would seem to confirm this scenario.

I was driving on a smooth country road. Some twists and turns. Nothing major at all.

I always check to see if there’s leaks by seeing if the front skid plate looks wet. In the past I could tell it’s leaking by noticing the front skid plate looks moist with fluid. Ever since they replaced the front axel it hasn’t been.

The truck runs fine in 2WD.

Also, when I engaged 4WD this time it was pulling hard to the left. It was difficult to maintain control at times.

you really shouldn't drive in 4wd on dry pavement unless you are going in a straight line, I also engage 4wd while driving to keep the system functioning properly, however I only do it on long straightaways and disengage if I'm expecting to turn at all, the clunking and hesitation to disengage was likely from significant binding experienced when driving in 4wd around turns on dry pavement

the good news is, even if you did wreck your tcase, they are such a pain to sell that they are usually available cheap and they are easy to remove and replace

Are all four tires the same size? Has the front or rear differential been replaced or serviced?

All tires are not the same brand. But are the same size. The front axel was replaced back in November. I don’t know if the center differential was serviced though. I will call the dealership and find out when they open tomorrow.

Different brand tires will be different sizes. This is your problem. All four tires must match in any 4wd/awd vehicle. The 4x4 system is binding due to the different size tires.

It's also possible that the front and rear differential gears do not match.

Really? Shit. Now I not only have a possibly busted transfer case I have to get two new tires. Ugh.

the primitive 4x4 system in the 4runner doesn't care if you have slightly mismatched tire sizes, don't worry about the tires

Primitive? ANY 4x4 will care about mis matched tire sizes.

Particularly on dry pavement.

In the non-snowy months I usually give the 4wd a little exercise on rainy days.

maybe if you have two different sizes, but different brands with slight differences in size? not really. it's not the best situation but it's not going to hurt anything

Different manufacturers vary in size greatly. Even if all four tires are the same brand/size but say the rears are worn and the fronts are brand new will be enough of a difference to upset the 4x4 system.

you should definitely run matching tires in good condition, but the OPs tire setup isn't what damaged his 4x4 system, it was the driving on dry pavement around turns with 4hi engaged. it's not like newer 4x4 and awd systems with clutch packs in the transfercase which are very sensitive to tire size and tread depth