Hi, I have a 1978 Ford F100 with F250 4wd running gear in it , it is a 4 speed, with a 302, we just got the truck about two weeks ago, and it had the front driveshaft out (always a bad sign) I locked the hubs, and the front diff. yoke wouldn't turn, all good, then I unlocked the hubs, turned the front diff yoke, it turned, and the axles turned, all good there. I bought some new u joints, put them in the driveshaft, installed it, and tried the four wheel drive, nothing! The transfer case shifter moves easily, and freely, I put the trans in neutral, shifted the transfer case into 4L, put the trans in reverse, backed up about 10 feet, then put it in first gear, rolled forward (this is Florida soft dirt) then got on it a little bit, back wheels spun like hell, no front wheel drive action at all, then I repeated the process in 4H with the same results. I had my friend watch the front driveshaft with the hubs unlocked, and the front driveshaft turns whether it is in 2H, 4H, or 4L, I guess that's just from friction? I can't help feel that there is a linkage problem inside the transfer case. the shifter feels like it is working.........I'm stumped! HELP !! Lol

 

The most likely culprit is the locking hubs. I'd take them apart, check everything and service them. I'm sure there's more than 1 write up on here. Let us know what you find.

 

OK, Thanks, I thought of the hubs first too, but before I put the driveshaft in, I locked the hubs, and tried turning the front diff yoke, and I couldn't, so that told me the hubs were working

 

It could still be the hubs. That NP205 is very hard to kill. Most often if it wont 4WD it wont 2WD either unless you have a broken output shaft after the bearing, but the yoke would fall out.
If one hub is broke, put it in 4WD it will not transfer any power up front unless you have a locker/ spool. Limited slip will not help there. But with your hand power the yoke will not move, you're exerting a few ft/lb TQ with your hand, nothing compared to the engine which will always find the weak link.

 

Pretty sure I found the problem, believe it is the front differential. I locked the hubs, put the transfer case, and transmission in neutral, grabbed the front driveshaft, and turned it, and it turned a little, then acted like it locked, then I grabbed it with both hands, and turned a lot harder, and it broke loose again, then I turned it the other way, and it turned a little, then stopped, and after more exertion, it broke free again, however, the axles are NOT turning, so the hubs are working, think I may have so some bad spider gears

 

YAY differential maintenance!!! It could also be your pinion gear. To replace all that it is not hard, proper tools help and a big breaker bar. Post pics!!! Good luck.

 

I wish I was as excited about as you are! Lol

 

I'd gladly trade you. You can fix my floors and I'll build your diff. The only good part of my cab floor is the transmission tunnel, for about a foot. Everything up to the firewall is toast.

 

Interesting, Floors are a lot easier, might be a good trade! ; Lol . Took the cover off today, the spiders are stripped, and the ring has a few chipped teeth, not one little piece ofmetal came out of the diff, so....the previous owner already had the cover off, cleaned it out, pulled the driveshaft, and lied his *** off! Can't win 'em all! Lol