I have a 2004 F150 XLT reg. cab, I just had it lifted with a 6" Fabtech and had 36x13.50xR17 IROK's put on. It is awesome, the only problem I have a shake from about 50mph to 60mph. I didn't have a steering stabalizer put on, could that be causing it? It's not real bad but its enough to notice.

 

That is a possibility.

 

a steering stabilizer may help but i bet it has something to do with your tires, i had to get mine balanced 2-3 times before i got rid of a slight vibe at 45-50

 

bigtruck has nailed it
I get the vibe between 65-75 with the dual stablizers. It is the tires. When I take it through the mud it seems to really pack in and I have them rebalanced about every 2 months.