Turn and 4-ways have different fuses
Turn signal is key on only fuse
4-way is full time 12v fuse
If flashing stops with key off and turn signal on then all 4 were running off the turn signal fuse
You can get your 1999 Owners manual here to ID those fuses:
https://www.therangerstation.com/manuals/
In 1995 and up there is only 1 Flasher unit, turn and 4-way share it
But with separate wiring inside so unlikely to be the issue, but not impossible
As said the Multi-function switch(turn signal stalk) is the crossover for turn and 4-way
Do the 4-ways work normally?
Key off and on
In 1999 there are 2 wires out of Multi-function switch, one for each side, the dash indicators are spliced to these wires
Turn signal should only connect 1 wire to the Flasher, the left side or the right side
4-ways connect all both wires to Flasher
If it was a bad ground out at one of the bulbs I would expect a report of one side being much dimmer than the other side, which you didn't notice
My bet would be Multi-function switch
EDIT:
If you have a Trailer light connector, especially DIY, then that could short BOTH wires together, and you would get what you have