I saw this question asked while researching the topic of front clip conversions and didn't see a definite answer to the question. After completing the entire front clip conversing on my 90 I thought I would post an answer. I installed 4wd fenders, corners, bumper, and a new 2wd grill. My existing 2wd turn signals are too small for the cutouts in the 4wd bumper. Also, the mounting holes are too wide for the turns as well. I was able to get one screw started on each turn signal and hold the other side up with a zip tie until I can order some 4wd turn signals. Hope this helps someone.

In the pic below you will notice the turn signal is fitted into the top right corner of the slot.



Edit: my truck is a 90 and the 4wd bumper was advertised as a 89-91 4wd bumper. Other years may differ.