Any pros or cons to swapping to a solid axle in front on my 1990 4x4 f250 ext cab diesel. What years and axle would fit? It is the heavier front end. Rated at 8600lbs.

 

Do u need a straight axle? Or just want one? 85ish-97 f350 4x4 front axles will bolt into your 250

 

there isn't much advantage to the solid axle,other than it's much more rugged for off road use,a commercial plow truck,or something you want to lift and put big tires on.
if it's a primary DD/worker for the street,with mild off road/plow duty,then a D60 swap isn't worth the grand to swap imho.
the con is,the straight axle will ride firmer than the TTB.it sits higher,which means you raise the rear as well.this isn't a con to most.....least not until this energy crisis hit with record high sick fuel prices.the higher the truck sits,the more resistance and therefore lower economy to one that sits lower.
there are pro's and cons to every modification you do to a truck.you personally need to determine which outweighs the other for your use.
though many will disagree,it's not as black and white as a D60 is better than a D50.