WideMike:
"They've [ heavy/highway contractor] offered two (2) positions verbally, but "tap dance" around submitting offer letter(s)."
Everybody has a different negotiating style. Nothing should stop you from documenting the offer by sending the offerer a letter clarifying the offer and presenting your response, (ie. acceptance, rejection or counter offer).
"I've had the misfortune in the past of employers "modifying" verbal offers at a later time under a variety of guises, "forgot to tell you something at the interview", "something suddently came up", etc.
I take you literally when you say "employers", which can only mean you must be an employee, and therefore have an employment contract (written or oral) defining conditions of employment. It is quite natural for either the employer, or the employee to amend the terms of their contact from time to time, by adding or deleting conditions of employment. Any significant proposed change of employment conditions can, of course, be rejected, or accepted, with or without compensatory conditions (such as more money, health benefits, etc.).
'Any ideas on strategy?
Arrange all your negotiations to be in writing, even if you have to initiate all the documents yourself. This should make you appear that much more professional and rigorous. Make notes in a diary as soon as possible after a negotiating session. Arguments over individual memory will inevitably favour the better documented position.
"Is explicitly asking for an employment contract advisable or not?
Whether oral or in writing, you still have an employment contract with defined conditions. Unless it is in writing, you may find it difficult to prove the conditions to anyone else.
Regards,