Replaced the starter but while the engine is on the starter wont disengage. Burnt up 2 starters doing this! I've heard it may need to Shimmed, or that the battery cables may need replaced... Anyone know???

Why did you change the starter in the first place? The battery need a full charge for the starter to operate properly, Need year make and model for more help. If the starter is grinding now and wasn't prior to replacement, it may have to be shimmed or the starter could be defective.

This is usually a mechanical issue. It is caused by either

1. The starter not being properly shimmed out on installation and the starter teeth are getting hung up in the flywheel and won't release after starting.
2. The return spring in the starter solenoid has broken and won't retract the starter gear.

If the starter has been in the vehicle for a while and has worked well, then #2 is more likely. On rare occasions, an electrical problem could be keeping the starter engaged, but this is not the norm.

Typically this sounds like a chewed up ring gear or flex plate but if you recently replaced that, the likely culprit is the pinion gear on the starter. It could be chewed up or the starter is not throwing it out enough. Also could be that the starter isn't properly shimmed.

Replace the ignition switch

sounds like it needs to be shimmed

possibly ignition switch is sticking then vs the starter bendix sticking

Since no one has answered yet, I will comment. When any of these starters turned, their bendix switch should have extended to catch the flywheel. On some, a pin is used to position the bendix on the starter shaft.

Can not say why original bendix does not work except that it must be pinned to the shaft. For replacement starters, there can be differences in the gear size of the bendix. Even some gears have different amounts of teeth on the bendix gear. Have not heard of needing shims to mount the starter to the Ford engine, but old GM products were shimmed. For that matter, some windings inside starters could be reversed, especially in Marine dual engine applications.

You went to a lot of work to change the flywheel, check the length of the starter to see if it is mounting deep enough to catch the flywheel. Maybe the bell housing is interfering with installation.

It depends on what the noise is. Normally when there is a sound heard from the starter it is the solenoid going bad. Or, if the starter was replaced, it may not have been shimmed properly. Does the truck still start? And, what year and size of motor are we talking about?