I have a 08 f350 that I put led tail lights in,now the signal blinks fast like there is a bulb out?Do I need a heavier flasher?

Hey Ronald, Try reseating the LED Bulbs. Depending of the bulbs a polarity change might cause them to flicker faster.
The original bulbs were working off a flasher or relay already. Put Hazard signals On see if the same thing happens. Or simply get the original bulbs back in.
You can also try calling the manufacturer of the LED light bulbs with the model number.Let me know what happens.
Good Luck,
JP
PS I have added a link that may help you. https://www.superbrightleds.com/blog/led-turn-signals-blinking-too-fast-hyperflashing/275

LED's have resistance than incandescent bulbs, that causes a problem, so best thing to do is get ones that come with the correct resistors.

I would suspect that it has something to do with the flasher unit. They operate based on current draw heating up a bimetal piece that causes the blinking. Four-ways (hazards) draw more current since there are more filments/LEDs drawing current. By changing from incandescent to LED you have reduced the current draw on the flasher. You may need a different flasher that uses lower current to get turn signals to work. Here is a link that explains it and provides info on how to get the proper flasher.


LED Turn Signal Bulbs Flashing Problems

You may have the wrong bulb installed. Although they mostly look the same, some have different configurations.

I think your flasher is working properly if its blinking fast while that light is out. I think you may have a grounding issue with the wires on that light. Unplug it again and look all down the wires to see if there are any cracks or brakes. You can take the bulb out and attach the positive lead to the bulb and ground it to the chassis. It should turn on. If it doesn't then your problem is in the positive lead, if it does turn on your problem is with the ground wire.

the fuse is broke

NO PROBLEM TURN ON THE RIGHT BLINKER THEN WALK AROUND THE CAR MAKE SURE FRONT AND REAR BULBS ARE FLASHING... IF YOU HAVE A BULB OUT YOU WILL NOT OFFER ENOUGH OF A LOAD TO THE FLASHER... WILL CAUSE IT TO FLASH FAST... IF OK THEN MAKE SURE THE PROPER BULB NUMBER ON BULBS AND MAKE SURE CORRECT BULBS USED IN VEHICLE.... IF OK CHANGE FLASHER

This typically occurs when a turn signal bulb is out on the side blinking fast.