I've got a 96 f150 4.9L f150 with a 4x4 swap from a 92 f150 4.9L. I did the swap last summer, and when I did it, the 4x4 transmissions shift forks were all crap and wouldn't shift into 1st or 2nd or 5 and reverse. I took the top plate including shifter and shift forks from my 2wd m5od and put it on the 4x4 transmission, and it worked fine except for have to apply a bit of pressure to keep the reverse in reverse and not pop out of gear however not a significant issue stopping me from being able to drive.. anyways it's been like that since August, hasn't seemed like its gotten worse or anything, just got used to it and no big deal.

Today however, I was up in the mountains in the snow cutting a christmas tree, got stuck, went to back up and the transmission would not even engage in reverse. after a bit, I could get it to catch and feathering the clutch real slow i could get it to catch and get out maybe 3 or 4 feet before losing its catch on the gear and spinning and grinding, anyways, got out, used reverse as minimally as possible... 1st and 2nd worked fine, although it was definitely stiffer trying to get into gears than normal.
Then, trying to get out onto the highway back in 2wd (hubs still engaged) I pulled out, accelerated and the transmission wouldn't go into 2 or 3 at all even with jiggling, adjusting the clutch but went into 4th and just lugged my way up to speed. Once we got out of the snow I pulled over, unlocked the hubs and it shifted fine through all the gears and I drove 60 miles home completely fine, shifted great through all 5 gears.. reverse was back to normal with having to hold it in reverse. when I got home I locked the hubs again and it would do the exact same thing in reverse, however not quite as bad, I was able to find reverse and get it out but I feel like this might have been because it was on flat ground with no real load on it.

Sooo any ideas? think I'm looking at a full transmission rebuild? or possibly just the 5th reverse forks? It just seems weird that it was so much worse in 4wd? It hasn't done that that bad the three other times I've had it in 4wd, and it was a little worrisome being on a tiny forest service road with no turn around and no reverse. Luckily I was able to put it in neutral and roll back down the trail very slowly..
Thanks

 

The fact that you only have the problem when in 4WD should be telling you something, but I'm not sure what that something is. The fact that things aren't 100% right even in 2WD mode is of concern as well. Are you sure that the 2WD and 4WD cover/fork assemblies are the exactly same and thus there's no issue doing the swap you did?

 

yeah the transmissions are the exact same on the engine side, the only difference is the output shaft and tail housing are made to bolt up to the transfer case.

 

just an update, it is no longer only in 4wd. It grinds in 2wd too now.
any ideas or words of encouragement before I start looking for a rebuilt transmission?
Is it possible that it could just be an issue with reverse that could be fixed without an entire transmission rebuild?

 

Are you sure the replacement cover/forks assembly was in good condition?

Regardless, I'd start by pulling that and inspecting.

My best guess is that the transmission got run too long with worn forks and bearing/synchro damage occurred. But that is just a WAG.

 

Yeah it's worn out. The reason it only did it in 4wheel drive was because that's a greater load on the transmission than when in 2 wheel.

 

That makes sense. If you've got a U-Pull-It junkyard near you, that may be a better bet for you than rebuilding your current gearbox. Admittedly there's some risk in going that route, but I've sourced ~10 transmissions from the junkyard over the years and they all were fine - just need to pick the good ones and leave the others behind.