F250 rear-end fit in an F150?
 

Here's my predicament. I changed the front and rear stock gearing (4x4) on my F150 (2001 SuperCrew) several years back to .410 gears to allow for better towing perf. Now I am getting a big "thunk" inthe rear end which feels like the limited slip differential is going out on it. The shop that did that work it OOB now. I am thinking that F250 rear ends come stock with .410 gears and I could get one from a junk yard cheap and just replace the whole thing vs. pulling the rear-end apart. What kinda trouble am I getting into?

Be prepared to relocate the spring perches, shock mounts and figure out how to make those brakes play nicely with the ABS in your truck.

OR

You could grab the 10.25" rear end that came in the F150's with the heavy duty package and the light duty F250 (that looked just like an F150). I believe they were 7-lug rims, but the rest of the axle should be a bolt-in affair; later models were a 4.10 ratio, although I am not sure about the axles from your body style.

Either way you go, you are going to have mis-matched bolt patterns between front and rear. With the 7-lug rear axle, I can only imagine that 4x4 versions of the truck came with a 7-lug front axle, leading me to believe that you could convert the front end to 7-lug as well. The major downside there is finding rims that are offered in the 7-lug bolt pattern.

Personally, I think your best bet is to find a different shop to assess and repair the dmage in your existing axle.