reverse won't engage
 

The truck is a 99 f250 ld with 126k on engine and 75k on the tranny. It has reverse but it won't always engage. usually if i drive a little ways in 1 and shift to reverse its instant, but it won't always shift. if i rev it to 12-1500 rpm it will engage? Help!!

Have you changed the fluid since installing the tranny?

the last tranny fluid/filter change was about 10k ago

also there is no check engine light or flashing od light, but the fluid level is good and the reverse lights do come on

also there is no check engine light or flashing od light, but the fluid level is good and the reverse lights do come on and the forward gerar work fine

also there is no check engine light or flashing od light, but the fluid level is good and the reverse lights do come on, and the forward gears work fine

You want to make sure the trans range sensor is in range. If you have access to a device to watch PIDS, monitor TRS or MLP and make sure it is not jumping.
Some things may be worth checking that can contribute to the symptom.
Low fluid level causes delayed engagement, worse cold.
Shift cable adjustment, sensor adjustment.
Range sensor failure to register proper gear selection.
Low pump pressure.
Low/Reverse clutch issues.
Main control issues.
Internal trans issues.

thanks roser, i'll check that out. sorry about the multiple posts my computer is a pos