Will the rear end and front end from a 95 f-250 4x4 with a power stroke bolt right into a 91 f-250 4x4 with a 351W

 

Yes. both sterling 10.25s in the rear.

For the front they are both TTB. But you did not say if the 91 was a super cab or not

95 has Dana 50 TTB
91 will have a Dana 44 TTB if reg cab, D50 TTB if super cab

 

91 is regular cab...does this pose a problem or is it just an upgrade

 

It would be an "upgrade" so to speak. But not really worth the effort to swap one TTB with another

 

Reason for wanting to swap is I now have 3:55 gears and the 95 has 3:73's was hoping would be easy swap as I do want 3:73 gears and can get them cheap

 

Unless someone did a gear swap, the 95 has 4.10 or 3.55. They are the ONLY two gear ratio options in those years.

Also swapping from 3.55 to 3.73 would hardly be noticeable. As in less than 150 RPM change unnoticeable