Hey all, I dug around in the forum, and didn't find anything related to my problem, so I thought I'd ask for input...

'91 f-150 4.9I6 8.8 rear end with the M5OD - currently sitting at 128K miles. Up to this point, I have not ever had a problem with the transmission. Runs fine, always has. Actually very pleasing on the freeway. Anyhoo, had to replace the clutch and slave cylinder right around the 100K mark, everything back to normal.... Then the slave cylinder started leaking @127k, wouldn't engage 3rd and had to take it to a (very reputable) shop a friend works at. Replaced w/11" clutch and slave cylinder again and machined the flywheel because they were already in there(wasn't really much more expensive for this, pretty much all the $$ is labor, stupid internal slave cylinder), replaced one of the shifter forks due to wear, rebuilt the shifter, replaced the clutch master cylinder as it had never been done, replaced the shift pedal rod everyone talks about, various bearings and bushings along the way, new rear shaft seal and u-joints... Nice side of this is we were able to have all the gears inspected (looked em over myself and I'm now a firm believer in the DexMerc fluid for manual trannies)... Here's where it gets... weird. In test drives, the truck will not 1)Start in first without hopping across the shop 2)if started in nuetral will not disengage first or reverse. With the truck off, you can hit all five gears normally. Except for the 1st and reverse problem, the truck has been driven and everything feels really good. What's confusing is that this is not a problem I had before. The techs dropped the tranny, pulled everything back out, replaced the clutch and slave cylinder with a the recommened (LUX? Does that sound right?)replacement, and after they put everything back together, still finding the problem with 1st and reverse. Techs are stumped. Tranny guys next door are stumped - although one of them has the same tranny in his '93, and he said "sometimes it just does that."

Any suggestions/imput?
(I noted the new clutch was 11" - an 11" is what they took out)

 

That is a weird problem. I'm thinking it has to do with the top cover being removed from the trans. I'm sure you already checked this and it's been a number of years but I recall the gear clusters had to be lined up just right for the shift forks to go into the right space. The Ford shop manual says:

Place synchronizers into neutral gear position. Make sure that shift forks on top cover assembly are in neutral gear position.

Position top cover to transmission case, and carefully engage shift forks with synchronizers.

 

This definately sounds like it's related to the shifter forks. Can you try swapping a good used shift tower on there? You can access the shift tower from inside the truck, with the transmission installed, if you remove the carpet and the trans tunnel plate. Sounds like first is remaining partially engaged. I'd pull the shift tower, set the shift hubs to neutral, make sure the new fork isn't galling on the hub, and put the shifter back with it in neutral. I've heard people say use first or neutral, neither one matters as long as the gears and shifter match.