I have a 98 f150 with a 4.2L V6 and a 5 Speed M5R2 manual transmission, I recently got a rebuilt trans for my truck. I installed it with no issues, while my truck was still on jack stands I tested it out, it shifted into every gear no problem but after I took it off the stands and pulled it out of my backyard, when I take off in 1st its fine with the clutch being slowly released, but as soon as I let go of the clutch the truck just buckles and some times tries to flat out stop, I'm not sure if I have a Clutch issue, Transmission issue or axle issue, when I tried it out on jack stands I had the stands supporting the rear on the axles so some of the weight of the truck was and the rear axle and it didn't buckle then so I wouldn't think it'd be the axles but when it was on the ground it'll buckle. I even had someone move the truck forward and release the clutch slowly until in was fully releases and looked underneath and saw the axle buckle upward, I have no idea what the issue is it only does that when the clutch is released or if I'm lucky enough to release it without buckling once I go to shift to second and press in the clutch the whole truck just comes to a complete stop with out the engine stalling. anyone have any Ideas? I thought maybe it was a clutch issue and that maybe I put the clutch plate backwards(which I'm pretty sure I didn't)but even if I did, I wouldn't even be able to shift it into any gear, at least that's what I think. I bleed the clutch before taking it out so I don't think that could be the issue either. anyway, if you have any idea what my issue could be, please let me now I'm trying to get my truck running asap. Thanks in advance!!!

 

"Buckles"....do you mean "bucks" as in jerking, jumping etc?

Also, please use paragraphs when posting. It's a lot easier to read, and when answering something like this, we may need to address each issue or step separately.

OK, before your replaced your transmission, did the truck operate normally?

You say the truck will move when in gear and the clutch is let out, but it seems like you are saying it stops moving when you are shifting to 2nd?

The axle movement you describe as "buckling". Do you mean the axle twists with the pinion rising or lowering?

A backwards clutch disc could be involved.

Do you get grinding noises out of it?