I have a 1990 F-150 4x4 that has a 4" susp lift and 33" tires... i have an 8.8 rear end with 3.55 gears and want to go lower,,  i've found a 1977 ford with a ford 9" rear end with 4.11 gears in the rear with detroit locker, and 4.09 in the front.. will the rear end out of the 77 fit in the place of my 8.8 without any engineering? 

I was also wondering what kind of special modifications would be required to get rid of my I.F.S. and replace it with the solid axel out of the 77 that has 4.09 gears in it?  What would be the easiest way to swap those out if they're compatable?

It will fit the spring perches are the same, also take into consideration for the speed sensor in the rear axle, it runs the abs, speedo, computer controled speed sensors ansd sometimes the cruise

The rear end swap shouldn't be a problem. I did the exact same swap on my '91 F-150 about two months ago. The speed sensor in the rear shouldn't be a problem. It will just inactivate your rear ABS and the light will stay on in the dash, if yours is like mine anyways. The speedometer sensor should be in the tailshaft of the transfer case. I'm researching on the solid-axle swap for the front myself. 6 inches of lift on a TTB suspension sucks.  

we did a complete front and rear on a 94 f-150 we used 1977 44 frnt and 1977 9'' rear and used a 4'' lift kit you will need to CUT off the ifs center brackets so the diff can "up"travel and the 94 does not use a tail shaft speedo so i was checking into adding an older extention houseing with the speedo hole,just havent done it yet the truck will also need the radius arm bracets moved back for the longer radius arms(we used arms from 77 doner truck)all works well and went on in about 4 nights