dodge

I got a 91 dodge150 with 318. I want to boost power but i got a very tight budget. How can i get some more power without rebuilding motor  

To without using any power adders... Not what every one thinks is cool n sexy, but replace everything fluid with modern equivalents/rer; Synthetics. in the axles, t-case if possible, motor oil,etc. then you can maybe restor some parasitic power losses.
Now, the totaly UN SEXY part. taller skinny narrower street or an efficient A/T tread on the tire. if your running a 31 inch tall tire, ahh...265/70/16 or so try a taller to a point 32-33 inch tall STREET tire..read michelin x type tire.
or..
try a K/N replacement air filter. just the filter. maybe run a 3 inch singlr pipe to the back with a low restriction muffler.
electric fan(depending on use) for the street mild off road type fishin wheelin.
wax the pup, smoove airflow n all.
get rid of all extranious appendiges, off go the over cab lights( i know ugghh) ranchhand bumpers and other heavy duty stuff, unless ya really need them.
Lighter aluminum rims?
find the tourque peak of the 5.2 and regear for the bottom at 55-60 mph so's that the motors going at peak efficiency.
now, use good gas..
dont listen to the stupid argument that the motors dont need mid grade or supreme grade gas. the extra pennys you save are going down the drain as the suck gas makes the engine work harder.
try a experiment. fill up with mid for 2 weeks and chrt the milage/ then with low grade. your going a little further with midgrade.
luuck to ya and hope ive offered some solution  

Yeah, without tearing into the motor, you're pretty limited.  I think Dodge is on the right track with new gears.  Chances are, if that thing is an auto, you're running like a 3.55 ratio.  If you have an overdrive, go to 4.11s.  If you're just running a 3spd auto, then step up to 3.73s.  Also look into some good headers to compliment the free flowing exhaust that Dodge advised.  Headers will help out alot.  Once you open up the exhaust, you can look into things like larger air flow sensors and maybe even bigger injectors (it is fuel injected right?).  Maybe even a custom tailored chip for the computer from someone like Jet performance.