I am a little confused on this issue. Truck drives fine in 2wd as well as 4wd, but once there is a load on it in 4wd it just revs and no tires spin at all. Just like i am in neutral.
- The incident was backing trailer up a grassy hill after the rain. I try it in 2wd first (i have the Detroit trutrac in the rear) get close but the grass is too wet so i just start spinning the rear tires and trenching out the grass. So i pull forward and engage in 4wd. Shifts fine, i pull a bit more forward, no issue, and back up, no issue. Once i fall in the rut i made and there was some resistance the truck just revs up, no tire movement or slipping at all. As if i put it in neutral. So i put it in drive and it moves just fine (going down hill).

- years ago i was in the sand dunes and a van was stuck so i hooked up to it and tried to pull it out. Same thing happened the second the rope was tight and there was resistance it just reved up and didnt even spin a single tire an inch.

170k on her

Thoughts?

 

Sounds like clutches in the trans are slipping..... Wouldn't hurt to check the fluid, see what it looks like, and what interesting things you may find in the pan. 170K on the original trans is pretty good though. Wouldn't surprise me if it wasn't rebuild time....

 

I was thinking the same thing, I was just lost at why only under load or against resistance in 4wd. Fluid is good level and color, I am starting to get a shudder at takeoff which i believe is a failing tq converter. Rebuild time is fine, just curious if anyone knew the mechanics better than me.

 

Only 4wd? Range shifter fork pads can wear or be missing, transfer case might jump out of gear under load. Likewise if the detent on the manual case shifter is somehow monkeyed up. Shifting one way or the other resets the problem. Could be in the synchro sleeve as well.
No mechanical noises? Maybe not the issue. My 241 case jumped out after being well and truly stuck last summer, thought I had junked it after abusing the trans/case getting truck out.. Turns out a couple of the shifter pads had failed, sounded worse than it was when happening. Good luck.

 

Yeah I kinda doubt this is a t-case issue. If the range fork pops into the dead zone that is N you'll typically hear grinding and it won't readily return to engagement, plus the range fork isn't involved in the shift from 2hi to 4hi (although it's still responsible for holding the vehicle in high range at all times).

I've had BW1356s and Chevy NP208s pop out of gear due to a worn shift fork and it's a noisy, grinding affair

I think your trans is slipping. The additional traction of four wheels may simply overcome the holding ability of the trans clutches.